* Digression alert: My wife was shocked to learn Japanese eat the rapeseed plant because it was her understanding the plant, seed and oil were considered toxic to humans and livestock. She said that was probably why I had not seen flowering buds in the grocery store. I was sure "Nanohana" is related to rapeseed and has been eaten in Japan with no ill effect for quite some time so I decided to do some research.
Turns out, rapeseed contains glucosinolates which gives it a bitter taste and, in high doses, is toxic. The seeds of rapeseed apparently contain a higher level of glucosinolates than the leaves and buds. In addition cattle that were fed rapeseed meal (residue left over after the seeds were crushed for oil) didn't appreciate the taste, changed their feeding habits and lost weight further leading to the perception that it was toxic. Many vegetables such as broccoli, cauliflower, and Brussels sprouts, however, also contain small amounts of the same substance which is the source of bitterness in their taste. Rapeseed also contains erucic acid. Between the glucosinolates and erucic acid, rapeseed was not considered suitable for human consumption or cattle feed in Canada, US and the European Union until the 1970's when the Canadians came up with a rapeseed cultivar low in toxin and acid called CANOLA (CANadian seed Oil Low-Acidity). There is now a rapeseed cultivar (rapeseed 00) which has much lower erucic acid and glucosinolates and is considered fit for human consumption.
Japanese "Nanohana" may have its roots in ancient varieties of rapeseed which came to Japan from China centuries ago. It originated from west asian and northern european varieties that grew as weeds in barley fields. Also, since the leaves, which are eaten in nanohana contain lower concentration of glucosinolates it would be more acceptable for human consumption in Asia than in Europe where primarily the more toxic seed was use for oil production. Japan produced its own low glucosinolates and erucic acid cultivars as well as cultivars that are more suited as edible vegetables than for oil production. Nonetheless the vast majority of currently grown varietals in Japan are imported from the West. That was probably more than you ever wanted to know about the rapeseed plant.
As far as I can tell, we have two possible substitutes for Nanohana; one is broccolini and the other is broccoli rabe. Broccoli rabe, which is also called "Rapini" has a more assertive "bitterness" than "Nanohana" and broccolini, although similar in form and texture, has a very neutral taste and lacks the bitterness. I think, Rapini may be closer to Nanohana because of the bitterness. I have no idea which of these substitutes is more closely related to "Nanohana" taxonomically.
I made two small dishes; one from broccolini (below) and broccoli rabe (the second picture below, two separate evenings) to represent spring.
Broccolini: I used only the top portion with flowering buds. The long stalks are edible but tend to be a bit hard. I boiled it in salted water for 4-5 minutes or until the thickest part of the stems were cooked but still crunchy. I shocked it in ice cold water to stop the cooking and maintain the fresh green color.
Broccoli rabe: Similar to broccolini in terms of the preparation. I removed the larger stems and leaves especailly the ones that started turning yellow. I blanched it in the same manner as the broccolini including shocking in ice cold water.
Dressing: I had several choices; mustard soy sauce (karashi-zouuyu 辛子醤油) and sesame soy sauce (goma-shouuyu 胡麻醤油). Another choice is "ohitashi" お浸し meaning to "soak". For this preparatio, the vegetables are "soaked" in a mixture of dashi, soy sauce and mirin in 8-6:1:1 ratio for 5-10 minutes before serving
For the broccolini, I used mustard soy sauce. I put prepared Japanese hot mustard or neri-garashi 練り芥子 (1/4 tsp or to taste) in a small Suribachi すり鉢 or a Japanese mortar and added sugar (1/4 tsp). I added soy sauce in small increments as I mixed the mustard paste, sugar and soy sauce together using a pestle. I tasted it as I went until the combined sweetness and hot mustard taste was appropriate (about 1 tbs of soy sauce or slightly more).
For the broccoli rabe, I decided not to use any of the choices listed above and instead made an altogether different dressing. My wife had roasted walnuts for another dish and there were some leftover. So I decided to make a walnut soy sauce dressing. I ground about 2 tbs of roasted walnuts (dark skin removed by rubbing in paper towel) in a Suribachi Japanese mortar. When the walnuts released oil and became a bit pasty, I added sugar (1/4 tsp) and soy sauce (2-3 tbs). I also added mirin (1 tbs).
I simply dressed the blanched broccolini with the mustard soy sauce and garnished with roasted white sesame seeds. I dressed the broccoli rabe with the walnut soy sauce and garnished with coarsely chopped toasted walnuts as shown below.
This is a nice small dish to start the evening. It is nice enough substitute for nanohana. The broccolini lacks the distinctive slightly bitter taste which is characteristic of "Nanohana" and broccoli rabe is closest to Nanohana.
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