Add the flour and mix it with the onions and garlic almost until a paste consistency begins to form. Whisk in the milk, one cup at a time. Whisk, whisk and whisk until there are no lumps. Bring to a boil and let the sauce thicken for about 5 minutes, continually whisking. Remove from the heat.
Add 1 cup of the cheese and let it melt into your sauce.