Trim about ½-inch of the tops off the bell peppers; discard.

Using a spoon, remove the seeds and membranes from the interior of the bell peppers; discard.

Place a pepper upside-down in a microwave-safe bowl. Cover tightly with plastic wrap. Using a paring knife, poke a few vent holes in the plastic wrap.

Microwave on high for 2 minutes. Carefully remove from the microwave, remove the plastic and place the bell pepper, open-side up, into a 3-quart casserole dish.

Pour out any liquid from the bowl and repeat with the remaining bell peppers.

Meanwhile, preheat the oven to 350-degrees.

Warm the olive oil over medium-high heat in a large skillet.

Add the onions and sauté until the edges begin to brown, for 4 to 5 minutes.

Add the garlic and sauté until fragrant, about 30 seconds.

Add the sausage and, using a wooden spoon to break up the meat into small pieces. Cook, stirring often, until it starts to crisp, for 8 to 10 minutes.

Add the diced tomatoes and marinara sauce, and stir, scraping the bottom of the pan to remove any browned bits.

Add the rice and oregano and stir to combine. Cook until warmed through, stirring constantly, about 2 minutes; Season with salt, to taste and remove from the heat.

Generously fill the bell peppers with the sausage and rice filling.

Cover with foil and bake for 10 minutes.

Remove the foil and sprinkle 2 tablespoons of the cheese on top of each pepper.

Bake uncovered until melted, about 10 minutes more; serve immediately.