Cut the peppers in half lengthwise and remove the seeds and membranes, set aside. Preheat the oven to 200C/400F.
In a large and deep pan heat 1 tbsp of olive oil over medium heat and add minced garlic, cook for 30 seconds while stirring, then add the ground beef, dried oregano, dried basil, fennel seeds and a pinch of salt. Cook until the beef is no longer pink taking care to break up the beef with a spatula.
Add the tomato passata or crushed tomatoes and water, then stir. Add arborio rice, stir everything to combine, bring to a boil, lower the heat to low-medium and cover with a lid. Cook covered for 20 minutes, check occasionally and stir. At the end of 20 minutes stir in Parmesan cheese, then taste for seasoning and add salt if need. At this point the rice should be cooked al dente and the mixture will have absorbed about ⅔ of the liquid. It will still be loose and a little runny.
Fill the pepper halves with the beef risotto and arrange them in a baking pan, cover tightly with foil and bake at 200C/400F for 40 minutes. Then remove the foil and cover the peppers with grated mozzarella, return to the oven and cook for additional 15 minutes until the cheese is melted and the peppers are soft.