I made this as a piroshki filling for a Russian-themed dinner party I threw last weekend. I had discovered the Pike's Place Market cookbook in the Los Angeles Central Library which had the recipe for the first and most memorable piroshki I ever ate, at a Russian cafe in the market when I was a child. The recipe for the filling, however, is my own, though the ingredients are typical for cabbage piroshkis. The onions are carmelized for 45 minutes until they become delicious threads of sweetness, and the cabbage goes in for at least another half an hour along with butter, salt, and pepper. The result is soft, savory, sweet, and addictive. Quite a bit of the filling went straight into my mouth and not in to the pirsohkis. If you do use this for piroshkis, you can make it while the dough is in its first rise. I recommend putting on a comforting black and white movie (Arsenic and Old Lace in my case) and absorbing the homey scent of carmelizing onions while getting up now and then to stir them. —clintonhillbilly
Slice your onions into short thin pieces, an inch and a half or two long.
Heat 2 tbsp canola oil in large nonstick pot. You want this pot to be big enough to hold several cups of cabbage as well as the onions. Add the onions, and sautee over medium heat for 45 minutes, stirring every few minutes. I found that the sautee function on my pressure cooker was perfect for this, and the surface was nonstick enough that I never had to add oil. The onions will become soft, then translucent, and then the will start to brown. Keep on going, they are not done yet. After about 45 minutes the onions should be thoroughly browned and very sweet and soft. This step takes patience, but it is SO worth it. If you want, make extra and throw them into mashed potatoes, stews, or anything else that could benefit from a delicious hit of savory-sweet.
While the onions are caramelizing away, chop up your cabbage. Remove the outer leaves and slice the cabbage in half. You'll then be able to see the core. Remove the core as best as you can. Thinly slice the rest of the cabbage and chop it up into small pieces.
When your onions have become truly brown and thoroughly caramelized, throw in the cabbage along with 4 tbsp of butter and season it well with salt and pepper. Toss with the onions, then put a lid on it and let it sweat. The butter will melt, and you should stir the cabbage around every five minutes or so. I continued on the sautee function, which would be medium heat, throughout this entire process, and occasionally took the lid off for a few minutes and then replaced it. Do this for about half an hour.
When you have completed this process, your cabbage too will be super-tender and caramelized. Try a bite and adjust the salt and pepper if needed. Serve as a side dish, or stuff your pirsohkis with it!