Spray a 13x9x2-inch baking dish 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 the baking dish layer half of the tortilla pieces, half of the leeks, half of the shrimp and crabmeat, half of the cheese, and half of the chopped dill. Repeat layers. In a bowl, beat together eggs, milk, flour, and pepper until smooth. Pour egg mixture over 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 12 servings.