Peel and finely dice the onion and garlic. Dice the olives as well
Heat the oil. Sauté onion and garlic until transparent. Steam the olives briefly. Add tomatoes. Bring everything to the boil, then let it boil down at low heat for 20-25 minutes while stirring occasionally. Season with salt and pepper. Leave to cool slightly
Toast baguette slices in a toaster or oven until golden brown. Spread the tomato and olive paste on the raclette pan, cover with cheese and bake in the oven