Did you know that if you add boiling liquid (e.g. boiling milk) to your 'roux' (butter and flour paste) to make a sauce it will not make lumps (or much less so). You must then still continue cooking it because if you don't your sauce will have that "flour" taste.
To thicken a meat sauce: Drain the liquid from the meat through a strainer into another pot. Keep the meat aside. Melt butter in the original casserole pot and when melted add flour (about 1:1 e.g. 2tbsp each). Cook over medium heat for 2-3 minutes without browning while stirring. Bring the pot with the liquid to a boil and then add the boiling meat liquid to the flour-paste while beating quickly. It will thicken the sauce nicely. Cook this gravy for about 5 minutes while gently simmering to cook the flour. Then return the meat to the casserole and stir through.