Preheat the oven to 400°F. Prepare a baking dish by lightly spraying with oil. Set aside.
Place sausage in a large skillet over medium-high heat. Using a wooden spoon or potato masher (my favorite tool for this task), break up the sausage and allow it to start browning.
Add the chopped onion to the pan and cook until the sausage is no longer pink and the onion begins to soften, about 4-6 minutes. Stir in grated or minced garlic and cook for another minute.
Add marinara sauce (or crushed tomatoes) and ¾ cup vegetable or chicken stock; stir to combine. Bring to a bubble and allow to simmer for about 5 minutes or until the liquid has reduced and thickened a bit.
Stir in rice until fully combined. Mix in mozzarella cheese and cayenne pepper. Taste and season with salt if needed.
Using a small cookie scoop or a spoon, fill each pepper to the top. You may have extra filling depending on the size of your peppers. See notes in post for other uses.
Place each pepper carefully on its side in prepared baking dish. Pour remaining ¼ cup stock in the dish -- this will help steam and soften the peppers as they cook. Cover tightly with foil and bake for 40-45 minutes or until peppers are tender.