For the recipe for gratinated fennel, cut the fennel bulbs in half lengthwise, remove the outer leaves and stems and keep the fennel greens. Boil about 1 liter of salted water with the lemon juice and cook the fennel in it for about 20 minutes. Remove, drain and keep the broth. Coat an ovenproof dish with olive oil. Drain the canned tomatoes, coarsely chop them and place them in the dish. Put the fennel halves in, drizzle with about 125 ml of fennel broth and season generously with salt and pepper. Finely chop the parsley, fennel greens, onion and garlic cloves. Sauté the onions and garlic in a little olive oil. Stir in the breadcrumbs and toast until golden brown. Remove the pan from the heat, mix in the herbs and Parmesan and spread everything over the fennel halves. Bake in the oven preheated to 200 °C for about 20 minutes.