Preheat oven to 400° F.
Line baking sheet with parchment paper. Set aside.
Heat medium nonstick skillet, lightly coated with spray, over medium-high heat.
Add onion and bell pepper; cook, stirring frequently, for 3 to 5 minutes, or until onion is translucent.
Add garlic; cook, stirring frequently, for 1 minute.
Add spinach; cook, stirring frequently, for 2 to 3 minutes, or until
spinach is wilted. Set aside.
Combine eggs, ricotta cheese, and Parmesan cheese in a medium bowl; mix well.
Add onion mixture, basil, and pepper; mix well.
Place mushroom caps on prepared pan, gill-sides up.
Stuff mushroom caps with cheese mixture; using the back of a spoon, make a well in the middle of the cheese mixture in each mushroom.
Fill each well with 2 Tbsp. marinara sauce and top with 1 Tbsp. mozzarella cheese. Bake for 24 to 26 minutes, or until mushroom caps are soft, and cheese is melted.
Place 2 mushroom caps on each serving plate. Evenly divide spaghetti squash between serving plates.