Peel the onions and slice or cut them into very fine rings. Wash the thyme. Pluck the leaves, except for a few for garnishing
Heat the fat in a coated pan. Fry the onion rings in it in portions while turning them golden brown. Add thyme at the end. Season with pepper and salt if necessary. Cut both cheeses into thin slices
Place slices of bread next to each other on a baking tray lined with baking paper. Sprinkle with onion rings. Place cheese on top. Bake in the preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/gas: level 4) for 5-8 minutes. Serve with tomatoes and sprinkle with the rest of thyme