Pour the tomatoes with their liquid into a frying pan and cook for about 15 minutes. Stir occasionally until the tomato mixture looks thick. Season with salt and pepper.
Meanwhile, line a buttered pie dish with the shortcrust pastry sheet.
Brush the pastry with the mustard, add the tomatoes, sprinkle with herbs and cover with grated Gruyere.
Bake for approximately 40 minutes until the shell is cooked and filling is golden brown.
