Melt the butter in a medium saucepan. Add your onions and saute until soft, about 5 minutes.
Add the flour. I often use unbleached flour for dusting and thickening if I only need a small amount. I have successfully used arrowroot powder/flour many times, though.
Stir this together, then add the milk.
Cook and stir on medium heat for a few minutes until the sauce is thickened.
Add your veggies to an 11x7 baking dish.
Top with sour cream.
Pour in the white sauce and a pinch of salt and pepper. Add the shredded cheese, reserving a little for the top.
Stir it all up and top with cheese.
Bake at 350 for 20 - 30 minutes, until bubbly and the veggies are soft. So simple and yummy!