When I made extra octopus legs to Galician octopus tapas, I also made a Japanese-style simmered octopus.
This is a rather simple recipe. I started simmering the octopus legs and head (body) in water with a dash of sake with an Otoshi-buta 落とし蓋 (see below) and a regular lid. I cooked it for over 1 hour before I seasoned the broth. I did not measure but added sugar first (1 tbs to about 3 cups of boiling liquid). I simmered it for 30 minutes and added soy sauce (about 1 tbs). I removed the lid and turned up the flame to low-medium and reduced the broth to 1/3 (took another 30 minutes).
The sauce became thick and clingy (The first picture). I cut the cooked meat into bite sized pieces and served. We like this preparation better than Galician octopus. The meat became really tender after long hours of cooking. With this dish we congratulated ourselves for completely finishing the whole boiled octopus.
P.S. Once the simmer octopus was refrigerated, it got chewy again. heating up in the microwave did not improve.