Peel and julienne the onion. Peel and cut the potatoes into slices about 1 cm thick. Wash and slice the tomatoes, leaving the skin on.
Crush the cloves of garlic, skin and all, using a pestle and mortar.
Grease an oven tray with extra-virgin olive oil and arrange the chopped vegetables in the following layers: potatoes, tomatoes, garlic, and onion. Add salt and pepper to taste, add the herbs and drizzle some more extra-virgin olive oil on top.
Cover the tray with aluminum foil and bake in the oven for about 30 minutes at 180 ºC.
Now add the salmon steaks, turn up the temperature to 200 ºC and cook for about 10 more minutes.