In a medium saucepan, melt the butter over medium heat, then whisk in the flour until the butter is absorbed by the flour. Turn the heat up to high, then add the milk and whisk to dissolve the flour (you're making a roux). Add the mayonnaise, mustard, nutritional yeast, lemon juice, salt, and pepper. Whisk continuously until it starts to come to a slow boil, then reduce the heat to low and let it thicken for a few minutes, whisking occasionally. If too thick, add more milk.
Serve over tofu scramble, potatoes, asparagus, or anything you’d like. Store leftover sauce in the fridge.