Vegetable stock

Name: Vegetable stock
Ingredients: 1 tbsp olive oil; ½ onion, diced; ½ leek, diced; 1 carrot, diced; 1 small bulb fennel, diced; 3 garlic cloves, left whole and gently crushed; About 20 black peppercorns; 8-10 button mushrooms; 1 stick celery, diced; 3 tomatoes, diced; 3-4 fresh parsley stalks, roughly torn