Place all the vegetables in a baking dish and pour 3 tablespoon of the melted cooking fat and mix well to combine.
8 plum tomatoes, ½ onion, 4 cloves garlic, 1 celery stalk, A handful each of fresh thyme, 6 tablespoon cooking fat
Roast the vegetables for about 5 hours in the preheated oven, until they are really caramelized.
Place the vegetables in a blender and process to a smooth sauce. Add the rest of the cooking fat, season to taste with sea salt and freshly ground black pepper and process again.