Peel and finely dice the onion and garlic. Cut olives into slices. Heat oil in a pot. Steam onions and garlic for about 3 minutes. Add olives and fry briefly. Add tomatoes and bring to the boil. Season with salt and pepper.
Reduce over low heat for 20-25 minutes, stirring occasionally. Leave to cool slightly. Heat half of the butter in a frying pan and fry the baguette slices on both sides until golden brown. Add the remaining butter in between and let it melt. Brush the baguette slices with the tomato and olive paste and place them in a raclette pan. Cover with a slice of cheese and gratinate