Preheat the oven to 375°F and lightly grease a baking dish large enough to hold all of the peppers snugly.
Slice the tops off the bell peppers, remove the seeds and membranes, and rub the outsides and insides with 1 tablespoon of the olive oil.
Arrange the peppers cut side up in the prepared baking dish and set aside while you prepare the filling.
Heat the remaining 1 tablespoon olive oil in a large skillet over medium heat until shimmering.
Add the diced onion to the skillet and cook, stirring occasionally, until softened and translucent.
Add the minced garlic and cook briefly until fragrant, taking care not to let it brown.
Crumble the ground beef into the skillet and cook, breaking it up with a spatula, until browned and no longer pink.
Drain off any excess grease from the skillet if needed to prevent the filling from becoming watery.
Stir in the riced cauliflower, drained diced tomatoes, and tomato sauce, mixing until everything is evenly combined.
Season the mixture with cumin, oregano, smoked paprika, sea salt, black pepper, and red pepper flakes, adjusting the salt to taste.
Cook the filling for a few more minutes, stirring often, until the cauliflower rice is just tender and most of the excess moisture has cooked off.
Remove the skillet from the heat and stir in the chopped parsley, then let the mixture cool slightly so the cheese will not melt immediately when added.
In a mixing bowl, combine the slightly cooled beef mixture with 1 cup of the shredded mozzarella and 1/4 cup of the grated Parmesan until well distributed.
Spoon the cheesy beef and cauliflower filling evenly into each prepared pepper, pressing down gently with the spoon to remove air pockets and fill them to the top.
Cover the baking dish loosely with foil, tenting it so the foil does not stick to the tops of the peppers, and place it in the preheated oven.
Bake the covered peppers until the peppers are just tender and the filling is hot, then remove the foil from the dish.
Sprinkle the remaining 1/2 cup mozzarella and 1/4 cup Parmesan evenly over the tops of the peppers.
Return the uncovered baking dish to the oven and bake until the cheese is melted, bubbly, and lightly golden on top.
Remove the stuffed peppers from the oven and let them rest for a few minutes to set before serving so they are easier to handle and slice.
Garnish the peppers with additional chopped parsley if desired and serve warm as a hearty low carb main course.