If you have the time to make it, it's well worth the trouble. And really, it's a lot like making broth. You need to be in attendance but for most of the cooking time you don't need to do very much.
Ingredients (makes 1 cup)
3 cups diced Spanish onions
1 cup olive oil
Kosher salt
6 plum tomatoes (1 lb)
1/2 tsp minced garlic
Instructions:
1. Combine oil, onions and a pinch of salt in a 8-9" wide heavy-bottomed pot over medium heat. When it starts to simmer, turn the heat down very low (for a gas stove, a diffuser is needed). Cook for 2.5 hours. The oil should continue to bubble gently and the onions should slowly stew, eventually caramelizing to a dark golden colour.
2. Prepare the tomatoes by cutting lengthwise and seeding. Then, grate (skin side out) against the large holes of a cheese grater. The pulp should go through and the skins should be discarded. You should be left with about 1 cup of tomato puree.
3. Add tomatoes to caramelized onions and cook for another 2-2.5 hours at the same low heat. When it's done, the mixture will sizzle and small bubbles will cover the surface and the tomatoes and onions will separate from the clear oil when stirred.
4. Remove from heat. Add a pinch of salt and the garlic. Allow to cool in the pan. Drain before using (Soffritto oil can be used to start another soffritto).
It keeps in the fridge for 1 week and the freezer for about 4 weeks.
Recipe from Thomas Keller's Bouchon.
