Spill chopped potatoes into a large pot. Cover with about an inch of water. Bring to a boil, then let them bubble away, uncovered, until the potatoes are tender and yield when poked with a fork or knife, about 15 minutes.
Drain in a colander. (Potatoes may be cooked up to a day ahead, then cooled, tightly covered and refrigerated. Remove from fridge and bring them back to room temperature for about an hour before proceeding.)
In a 10-inch skillet, preferably cast iron, melt vegan butter over medium-high heat. Add sliced onion and cook, stirring occasionally, until the onions soften and turn golden. Add chopped cabbage and give a quick sauté, stirring frequently, until the greens are tender but still bright, about 5 minutes.
Mix in potatoes, mashing and stirring them into the onions and greens. Go for lumpy and hashlike, not fluffy and uniformly mashed. Sprinkle in thyme leaves, if using, and season everything generously with sea salt and pepper.
Use a wooden spoon or spatula to press everything down to form a cake. Cook, without stirring, until potatoes squeak and underside is crusty and golden, about 10 minutes.
Spoon and serve at once. Or if you’re feeling adventurous, place a large plate over the top of the skillet and carefully turn out the potato cake, so the crusty side is on top. If it comes out in pieces, no worries, just press everything back into place. Then slide it back into the skillet and cook for another 5 minutes or so.
Eat. Enjoy.