This is a flexible dish, traditionally a vehicle for leftover meat. Vegetables work very well, and baked tofu would be good. The milk, of course, is whatever you normally use for milk. The fat can be vegetable oil, or butter, or the leftover fat from whatever leftover meat you're using.
For one serving:
1/2 cup cooked anything, cut into small dice
1 cup milk, warmed
1 Tbsp. fat
1 Tbsp. flour
2 slices toast, buttered if you like
In a small skillet, heat the fat. Add the flour and mix with a fork until there are no lumps. Let cook over medium heat for 1 minute. Add the milk in a thin stream, stirring with a fork the whole time. Cook over medium heat, stirring, until the sauce begins to thicken. Add the cooked anything and let it heat through. If sauce thickens too much, add more milk until it has the right consistency. Serve over toast.