This can be made in a saucepan on the stove top, just keep stirring so the sauce does not stick to the bottom, keep low to med heat.
Make a double mix for your lasagne adding basil and oregano. My preference is to make this first, it can sit aside in fridge until I'm ready for it.
To thin the sauce add more milk, to thicken add less milk.
My preference is to make sauce while dinner is cooking eg: corned beef. If sauce is finished first cover with a plate to keep warm, stir before use..