Preheat the oven to 375°F (190°C).

Cut the tops off the bell peppers and remove the seeds and membranes.

In a large skillet, cook the ground beef over medium heat until browned. Add the chopped onion and minced garlic and cook until the onion is translucent.

Add the diced tomatoes, dried oregano, dried basil, salt, and pepper to the skillet. Stir to combine.

Add the chopped spinach to the skillet and cook until wilted.

Remove the skillet from the heat and stir in the cooked rice.

Spoon the beef and spinach mixture into the bell peppers, filling them to the top.

Sprinkle shredded mozzarella cheese on top of the stuffed bell peppers.

Place the stuffed bell peppers on a baking sheet and bake in the preheated oven for 30-35 minutes, or until the peppers are tender and the cheese is melted and bubbly.

Remove from the oven and let cool slightly before serving.