4 large bell peppers (about 2 pounds)
2 cups small cauliflower florets
2 tablespoons extra-virgin olive oil, divided
Pinch of salt plus 1/2 teaspoon, divided
Pinch of ground pepper plus 1/4 teaspoon, divided
½ cup chopped onion
1 pound lean ground beef
2 cloves garlic, minced
½ teaspoon dried oregano
1 (8 ounce) can no-salt-added tomato sauce
½ cup shredded part-skim mozzarella
Preheat oven to 350 degrees F.
Slice off stem ends of bell peppers. Cut the flesh from the stem and chop. You should have about 1 cup. Scoop out seeds from the pepper cavities. Bring about an inch of water to a boil in a large pot fitted with a steamer basket. Steam the peppers until starting to soften, about 3 minutes. Remove the peppers from the pot and set aside.
Pulse cauliflower in a food processor until broken down into rice-size pieces. Heat 1 tablespoon oil in a large skillet over medium heat. Add the cauliflower rice and a pinch each of salt and pepper. Cook, stirring, until softened and starting to brown, about 3 minutes. Transfer to a small bowl.
Wipe out the pan. Add the remaining 1 tablespoon oil, the chopped bell pepper and onion. Cook, stirring, until starting to soften, about 3 minutes. Add beef, garlic, oregano and the remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook, stirring and breaking up the beef with a wooden spoon just until no longer pink, about 5 minutes. Add tomato sauce and the cauliflower rice; stir to coat.
Place the peppers upright in an 8-inch square baking dish. Fill each pepper with a generous 1 cup of the cauliflower rice mixture. Top each pepper with 2 tablespoons cheese.
Bake until the filling is heated through and the cheese is melted, 20 to 25 minutes.
To make ahead: Prepare stuffed peppers through Step 5 and refrigerate for up to 1 day.