Prepare for Baking: Preheat your oven to 375°F / 190°C. Lightly grease four 12-ounce individual casseroles or one 1. 5-quart baking dish.
Cook the Pasta & Veggies: In a large pot of boiling, salted water, cook the penne according to package directions. During the final minute of cooking, add the fresh spinach and quartered mushrooms to the pot to quickly blanch them.
Drain everything well.
Combine the : Transfer the drained pasta, spinach, and mushrooms to an extra-large bowl.
Add the sun-dried tomatoes, kalamata olives, basil pesto, and balsamic vinegar.
Toss gently until everything is evenly coated in the delicious pesto sauce.
Assemble the Casserole: Spoon the pasta mixture evenly into your prepared baking dish or individual casseroles.
First Bake: Cover the dish with foil and bake for 20 to 25 minutes, or until the pasta is heated through and bubbly.
Add Cheese & Final Bake: Carefully remove the foil and top the pasta evenly with the fresh mozzarella chunks or slices. Return the dish to the oven, uncovered, and bake for about 10 more minutes, or until the cheese is perfectly melted and golden.
Rest and Serve: Let the pasta bake stand for 5 minutes before serving. This helps it set up slightly. If desired, sprinkle generously with fresh basil just before digging in.