Preheat and Prepare: Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish to prevent sticking.
Make the Meatloaf Base: In a large bowl, combine the lean ground turkey, chopped onion, minced garlic, eggs, breadcrumbs, tomato paste, thyme, oregano, salt, and pepper. If using, fold in the spinach or bell peppers. Stir thoroughly until every ingredient is uniformly combined. Next, press the mixture firmly into the bottom of your prepared baking dish, forming an even layer.
Cook the Pasta: Meanwhile, bring a large pot of water to a boil. Add the elbow macaroni and cook according to the package directions until al dente. Drain the pasta and set it aside.
Prepare the Cheesy Sauce: Place a medium saucepan on the stove and slowly warm the olive oil over a moderate flame. Then, add the chicken broth and milk. Gradually stir in the cheddar, mozzarella, and Parmesan cheeses until a smooth and thick sauce forms. Be sure to stir frequently, and keep the sauce at a gentle simmer without boiling.
Assemble the Casserole: Toss the cooked pasta in the cheese sauce until every piece is well coated. Evenly spread the cheesy pasta layer over the meatloaf base in your baking dish. For a crunchy finish, you may sprinkle a little extra cheese or breadcrumbs on top.
Bake: Place the dish in the preheated oven and bake for 35-40 minutes, or until the casserole bubbles and the top turns a beautiful golden color. Then, remove the dish from the oven and let it rest for 5 minutes before serving.