Cook the beef in a hot frying pan with a little olive oil or avocado oil. Cook until browned and cooked through, about 7 minutes. Drain excess fat if necessary.
Add the onion and cook 5 minutes.
Add the garlic and cook 2 minutes.
Add the tomato paste, chili flakes, oregano, half the parsley, and fennel and stir to combine. Season with salt and pepper.
Remove from heat and stir in the nutritional yeast. The mixture should be pretty thick, but you can add a splash of water if it seems too thick.
Preheat oven to 350 F.
Put the zucchini onto a baking dish and distribute the meat mixture between the peppers. Bake for 30 minutes. Top with the remaining fresh parsley and serve.