In a pot, add the vegan butter and cook the onions in the butter until soft. Add flour and cook an additional 2 min. stirring occasionally.
Whisk in the milk and cook until the sauce is bubbly and thickens, stirring frequently.
Remove the pot from the heat and add 3/4 cup cheese. Stir until the cheese is all melted. Season with salt and pepper to taste.
In a glass casserole dish (be sure to use a shallow dish, preferably 9x13), place one layer of potatoes, sprinkle with salt/pepper, add 1/2 the sauce mixture, and repeat to form one more additional layer.
Sprinkle the top with the remainder of the cheese.
Cover with foil and bake at 350 degrees for 30-40 minutes or until the potatoes are tender and cooked (test with a fork).
Uncover, and bake for an additional ~10-15 min. until the cheese on top is melted and crispy-golden brown.
Notes
For vegan cheese, use Miyoko cheddar block or Daiya cutting board collection