Tuna, Tomato and Mustard Pie

Ingredients:

1 shortcrust pastry
2 to 3 tomatoes
2 eggs
1 can of natural tuna (about 150 g)
2 tablespoons mustard (Dijon or old-fashioned)
20 cl crème fraîche
100 g grated Gruyère cheese
Salt, pepper
Herbes de Provence (optional)