Cut the tops off the bell peppers and remove the seeds and membranes.
In a skillet over medium heat, brown the ground beef or turkey until fully cooked. Drain any excess fat.
Add the chopped onion to the skillet and sauté until softened, about 3-4 minutes.
Stir in the cooked rice, diced tomatoes (with juice), garlic powder, Italian seasoning, salt, and pepper. Mix well and cook for an additional 5 minutes.
Stuff each bell pepper with the meat and rice mixture, packing it down slightly.
Place the stuffed peppers in a baking dish. If desired, you can sprinkle the tops with shredded cheese.
Add a small amount of water (about 1/4 cup) to the bottom of the baking dish to help steam the peppers.
Cover the baking dish with foil and bake in the preheated oven for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the peppers are tender and the cheese is melted and bubbly.
Allow the stuffed peppers to cool for a few minutes before serving.