Heat the olive oil in a skillet over medium-high heat. Add the ground beef to the skillet and brown until almost cooked through. Add the onion and continue to cook, stirring occasionally, until the beef is fully cooked and the onion has softened. Stir in the enchilada sauce and cook for another 30 seconds, or until the mixture begins to bubble. Set the mixture aside.
Preheat your oven to 400 F degrees.
Slice the tops of the bell peppers about 1/2 and inch down, or enough to keep the tops fully intact. Set the tops aside. Use your hands to remove the seeds from each pepper.
Fill each pepper about 1/3 of the way with the meat mixture, a healthy layer of cheese, another layer of meat mixture to the top of the pepper, then a final layer of cheese. Place the tops back on each pepper.
Place the peppers in a casserole dish and bake for about 45 minutes to 1 hour, or until the peppers begin to blister and darken.
Please keep in mind that nutritional information is a rough estimate and can vary greatly based on products used.