Get Ready: Preheat your oven to 400°F (200°C).
Cook the Beef: In a large skillet over medium heat, brown the ground beef with ½ teaspoon of salt and the black pepper until it's no longer pink. Break up any large chunks. Stir in the minced garlic and red pepper flakes and cook for one more minute until fragrant. Drain any excess fat to keep it lean.
Prepare the Filling: Squeeze the thawed spinach really well to remove all excess water – this is key for a great texture! In a large bowl, combine the ricotta cheese, Parmesan cheese, the remaining ¼ teaspoon of salt, and the cooked beef mixture. Mix everything together until well combined.
Prep the Peppers: Slice each bell pepper in half lengthwise and scoop out the seeds and white ribs. Arrange the pepper halves, cut-side up, in a 9x13 inch glass or ceramic baking dish.
Stuff 'em Up: Generously spoon the hearty beef and cheese filling into each bell pepper half, creating a nice mound.
Top with Sauce & Cheese: Drizzle about 2 tablespoons of your chosen tomato sauce over the filling in each pepper. Then, sprinkle a generous amount of shredded mozzarella cheese on top.
Bake to Perfection: Bake for 30 minutes, or until the cheese is bubbly and melted, and the peppers are tender.
Golden Finish: For a beautiful golden-brown top, carefully switch your oven to broil for 1-3 minutes. Keep a close eye on them to prevent burning!
Garnish & Serve: Sprinkle with fresh chopped basil or parsley before serving. Enjoy this hearty and healthy meal!