Heat oven to 375 degrees F.
Trim off one thin lengthwise slice from each zucchini (about 1/4 inch thick) and reserve. Using melon-baller or small teaspoon, scoop out flesh and discard. Halve yellow squash lengthwise; scoop out flesh and discard. Sprinkle inside of shells with salt. Transfer zucchini and squash to 13 x 9 x 2-inch baking dish. Chop 2 reserved zucchini slices (you should have about 1/2 cup).
Heat oil in large skillet. Add onion and garlic; sauté over medium-high heat until softened, about 3 minutes. Add sausage, breaking into small pieces with wooden spoon. Cook until no longer pink, 3 minutes. Add chopped zucchini, Italian seasoning, salt and pepper; cook 3 minutes. Remove from heat. Let cool slightly, 5 minutes.
Stir mozzarella, Parmesan and cornmeal into sausage mixture. Lightly whisk egg white in a cup; stir into mixture. Mound 1/2 cup filling into each shell. Cover dish with foil.
Bake in 375 degree F oven 20 minutes. Carefully uncover dish. Bake another 10 minutes. Let cool slightly. Makes 6 servings.