Preheat oven to 400 degrees Fahrenheit.

Cut summer squash diagonally into 1/4-inch slices.

Season squash slices with 1/4 teaspoon of garlic salt and 1/4 teaspoon of black pepper.

Coat two baking sheets with cooking spray.

Place seasoned squash slices on the prepared baking sheets.

Bake in the preheated oven for 20 minutes.

Season plum tomato slices with remaining 1/4 teaspoon of garlic salt and 1/4 teaspoon of black pepper.

Spoon 1/3 of Italian-style diced tomatoes into the bottom of a large nonstick skillet.

Layer with half of the baked squash, half of the seasoned tomato slices, 1/3 of the diced tomatoes, and half of the Italian cheese blend.

Repeat the layering process with the remaining ingredients.

Top with grated Parmesan cheese.

Cover the skillet and simmer over low heat for 25 minutes.

Prepare whole wheat spaghetti according to package directions.

Serve the skillet summer squash parmesan alongside the cooked spaghetti.