Cut the top off the onions and peel them. Trim the bottom so it will lay flat. Score the tops crosswise to about halfway down being careful to not go any further.

Choose a heavy pot, either stainless steel, a Le Creuset type or ceramic. You want it to be a snug fit for all the onions to sit in a single layer. You'll need a heavy lid that fits well. You can add another onion or two if necessary to fill the bottom of the pot, but you will need to increase the amount of sauce accordingly.

Spread the kombu on the bottom of the pot and add 3-4 Tbsp water.

Arrange the onions top side up. Place pot on medium heat. Cover. Bring the pot to a boil and then immediately reduce heat to allow the liquid to simmer. Ideally, let the onions slow cook until they are meltingly tender. They are ready for the sauce when a paring knife slides in easily. This may take 40 minutes, or longer, depending on the size and freshness of the onions. In true nishime cooking, the lid isn't lifted during the cooking process so the energy can stay in the pot and go into the vegetables. This creates a super strengthening dish. In reality, I might lift the lid once or twice just to make sure the onions aren't running out of liquid and that it is still simmering. Find your own way with this. You don't want it to burn! You can use a flame deflector if that helps you prevent burning.

While the onions are cooking, mix remaining ingredients in a small bowl. Add a little water if needed to create a thick, spreadable sauce.

When the onions are tender, remove the cover and spread each onion evenly with the sauce. Try to keep the sauce on top of the onions so it doesn't slide off and mix with any liquid that is left. Replace cover and cook for another 3-5 minutes.

Remove from heat and serve.