Prepare Bell Peppers:
Cut tops off bell peppers and remove seeds.
Boil peppers in salted water for 3–5 minutes until slightly tender.
Reserve 1/4 cup of the cooking water. Set peppers aside to cool.
Cook the Filling:
In a skillet, heat olive oil over medium heat.
Sauté the Cajun mirepoix (onions, garlic, celery, etc.) until softened, about 5–7 minutes.
Add ground beef, season with salt and pepper, and cook until browned.
Stir in bread crumbs, grated Parmigiano Reggiano, and the reserved pepper water.
Cook for another 2–3 minutes, until the mixture holds together. Taste and adjust seasoning.
Stuff & Bake:
Preheat oven to 375°F (190°C).
Stuff each bell pepper with the beef mixture.
Place in a baking pan with olive oil, top with extra Parmigiano, and bake for 20–25 minutes, until golden on top and heated through.