Preheat oven to 350 degrees. Halve squash lengthwise; scoop out seeds. Place squash, cut sides down, in a large baking dish and prick skin all over with a fork. Bake until tender, about 30 to 40 minutes.
Meanwhile, cook turkey with season salt and pepper, stirring occasionally, in a nonstick skillet over medium-high heat until browned, about 5 minutes. Drain, remove from skillet and set aside.
In same skillet, heat oil over medium-high heat. Add onion and garlic and stir-fry until tender, about 5 minutes. Stir in diced tomatoes and Italian seasoning; bring to a boil. Reduce heat; add cooked turkey and simmer, stirring often, until desired consistency, about 5 minutes.
Place ricotta cheese and egg in a food processor or blender and puree until smooth.
Coat a 9-inch glass pie plate with cooking spray. Remove squash from oven and increase oven temperature to 375 degrees.
Using a fork, carefully rake stringy squash pulp from shell, separating it into strands that look like spaghetti. Arrange spaghetti squash strands in bottom and up sides of pie plate to form a crust.
Add ricotta cheese mixture and gently spread over squash.
Pour tomato-turkey sauce over cheese mixture and sprinkle with mozzarella.
Bake for 20 minutes.
Remove from oven and let stand 5 minutes before dividing into six servings.