Shrimp and feta cheese salad

1 lb Small Shells, Ditalini or other small pasta shape, uncooked
2 tbsp margarine
2 tbsp flour
2 cups milk
2 med tomatoes, chopped
1 cup cubed feta cheese
1 lb small shrimp, cooked
1 green bell pepper, chopped
1/2 cup drained black olives, pitted and sliced
1 tbsp chopped parsley
2 cloves garlic, minced
1 tsp oregano
1 tsp salt
1/2 tsp black pepper
Topping:
1/2 cup sliced almonds
1/4 cup fresh bread crumbs
1/4 cup grated Parmesan cheese
2 tbsp melted margarine

Prepare pasta according to package directions; drain. Melt margarine in saucepan; whisk in flour. Cook until mixture is bubbly. Whisk in milk. Cook over medium-low heat for 20 minutes, stirring often. Toss together pasta, sauce and remaining ingredients except topping. Spoon mixture into a 2 1/2-quart casserole dish. Cover and bake at 350 F for 20 minutes. While pasta is baking, toss together almonds, bread crumbs and Parmesan cheese. Mix in margarine. When pasta is done, remove from oven, sprinkle topping over and serve.