Combine the flour and milk together. Add sausage, soup, peas, macaroni, onion and sage. Turn into four 10 oz casseroles. Cover, seal, label and freeze.
Bake frozen casserole, uncovered in 400º oven 45 minutes.
For each casserole, melt 1 tsp butter and toss with 1/4 cup croutons. Sprinkle croutons around casserole edge. Bake in 350º oven for 5 minutes more (or top unfrozen casserole with buttered croutons, bake in 350º oven for 30 minutes).