Get your oven ready by preheating it to 400°F (200°C). In a large skillet over medium heat, brown your ground beef, seasoning it with a pinch of salt and pepper. Stir and break up the meat until it's no longer pink, usually about 10 minutes. Then, toss in the minced garlic and red pepper flakes and cook for just another minute until they smell fragrant.
While the beef cooks, firmly squeeze all the excess water out of your thawed spinach – this step is key for a great texture! In a large mixing bowl, combine the ricotta cheese, spinach, and Parmesan. Stir in the cooked ground beef mixture until everything is well combined and ready to fill.
Slice each bell pepper in half lengthwise, carefully scooping out the seeds and white ribs. Arrange these pepper halves in a 9x13-inch glass or ceramic baking dish. Generously fill each pepper half with the savory beef and ricotta mixture, mounding it slightly for a hearty serving.
Drizzle about 2 tablespoons of your chosen low-carb tomato or marinara sauce over the top of each stuffed pepper. Finish by sprinkling a generous layer of shredded mozzarella cheese over them – it's going to get wonderfully bubbly!
Bake in the preheated oven for 30 minutes, or until the cheese is beautifully melted and bubbly, and the peppers have softened to perfection. For a lovely golden-brown topping, switch your oven to broil and lightly broil the peppers for 1-3 minutes, watching carefully to prevent burning. Garnish with fresh chopped basil or parsley right before serving for a touch of freshness.