Preheat the oven to 350° F (176° C). (If you plan on baking it immediately.) Peel 4 large potatoes (or 6 medium potatoes) and set them aside.
Dice the onion into small pieces and saute it in 2 tablespoons of oil in a large wok. After 3 minutes, add 1/2 teaspoon of salt and 2 cloves of minced garlic and continue to cook over low heat for an additional 2 minutes.
While the onions are cooking, chop desired veggies into small pieces. (Or use a grater for carrots, summer squash, or zucchini.)
Stir in shredded carrots (or veggies of choice) and cook for 1 more minute and turn off the heat.
Use a mandolin to slice the potatoes into 3mm thick slices. (I do this overtop of the Wok.) You slice the potatoes by hand on a cutting board with a sharp knife if you don't have a mandolin slice.
Make the cream sauce
Heat 1 tablespoon of vegan butter or oil in the wok and sprinkle in 1 1/2 tablespoons of flour and stir well with a whisk to make a roux.
Slowly add 1 cup of plain plant-based milk to the roux a few tablespoons at a time stirring well between each to get out all of the lumps. (This will become very thick at first, just keep stirring and adding small amounts of more milk and it will thin out.)
Add 1 1/2 cups of vegan sour cream to the thickened milk along with 3/4 teaspoon of salt and 1/2 teaspoon of pepper.
Pour 2/3 of the cream mixture over the potatoes and veggies and mix well.
Transfer the mixture to a large casserole dish.
Pour the remaining 1/3 of the cream sauce over the top of the casserole.
Bake the casserole
Bake at 350° F (176° C) for 45 minutes then take it out of the oven and top it with 1 1/2 cups vegan cheese and put it back in the oven for another 15 minutes. (Place a cookie sheet overtop of the casserole for about 10 minutes after adding the cheese to trap some steam and help to melt the cheese.
Garnish with chives or green onions and vegan bacon bits.
Notes
Allow the onions to cook until they become translucent. Cooking them slowly will make them sweeter.
Slice the potatoes very thin. This helps them to cook faster. The more evenly you slice the potatoes, the more evenly they will cook in the casserole.