Preheat the oven to 190°C. Spread the breadcrumbs on a plate and leave them to dry a little while preparing the rest of the ingredients.
Cut the tops off the capsicums and scoop out the seeds and membranes. Blanch the capsicums and their tops in boiling water for 1 minute. Lift out the capsicums with a slotted spoon, then drain, cut-side down, on paper towel.
Heat the oil in a large frying pan over a medium heat. Add the onion and cook for 10 minutes, stirring frequently, until softened. Increase the heat, add the beef and garlic and fry, stirring, until browned and crumbly. Add the tomatoes, herbs and ¼ cup (60 ml) of the stock. Cook for a further minute, then remove from the heat and stir in the breadcrumbs. Season to taste.
Stand the capsicums upright in a lightly oiled, large ovenproof dish, fill with the beef mixture and press down gently. Sprinkle with the cheese and replace the tops. Scatter the fennel around the capsicums and pour in the rest of the stock.
Cover the dish with foil and bake for 40 minutes until the capsicums and fennel are tender, basting halfway through with the stock. Serve hot or warm.