Preheat oven to 400°F. You can prepare all steps in a large, oven-safe skillet (such as a 10 or 12" cast iron). If you don't have an oven-safe skillet, you'll need a skillet as well as a casserole dish.
Heat your skillet over medium-high heat and melt vegan butter. Add in sliced onions and sauté until onions are very soft and have taken on some color, about 8-10 minutes. If baking directly in the skillet, remove the onions to a bowl.
Layer about a third of the sliced potatoes on the bottom of your oven-safe skillet or casserole pan. Scatter half of the sautéed onions over top.
Create a second layer with another third of the potatoes and the remaining onions.
Top with the remaining potatoes. Pour the cream mixture over the potatoes. It should almost come up to the top, but it's okay if they are slightly exposed.
Optionally, place a few sprigs of your favorite fresh herbs on top. This will perfume the scalloped potatoes as they cook without overpowering the dish. I like using thyme.
Cover the casserole dish or skillet with foil and seal edges tightly in order to trap the steam to help the potatoes cook through evenly. If your skillet or dish is very full, I recommend baking it on top of a cookie sheet just in case it bubbles over slightly.
Bake in preheated oven for 45 minutes, covered. Remove the foil and bake, uncovered, for an additional 15-20 minutes, until the top is golden and the dish is beginning to brown around the edges. Test a few potatoes for doneness — they should be tender and easy to pierce with a fork or knife. If they seem a little undercooked, continue to bake in 10 minute increments until cooked.