Spray two 13x9x2-inch baking dishes with nonstick cooking spray. In a large skillet melt margarine or butter; cook leeks for 3 minutes or until tender. Tear tortillas into bite-size pieces.

In each baking dish layer one-fourth of the tortilla pieces, one-fourth of the leeks, one-fourth of the shrimp and crabmeat, one-fourth of the cheese, and one-fourth of the chopped dill. Repeat layers (each casserole should have two layers of ingredients). In a bowl beat together eggs, milk, flour, and pepper until smooth. Pour half of egg mixture over each strata; cover and refrigerate overnight.

Bake, uncovered, in a 350 degree F oven about 50 minutes or until a knife inserted near center comes out clean and top is golden brown. Let stand 10 minutes. Cut into squares and serve. Garnish with additional shrimp and fresh dill, if desired. Makes 24 servings (each casserole serves 12).

Make-Ahead Tip: Up to 24 hours ahead, prepare casseroles. Cover and chill. To serve, bake as directed.