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In a heavy-bottomed saucepan or le Creuset, heat the olive oil over medium and add the chopped onion (or leek and shallot) and thyme/bay leaf. Sprinkle a little salt on top and sauté over medium heat, stirring occasionally, for 10 minutes or so. Make sure not to brown the onions. Add the tomatoes, raise the heat a little, and cover. Cook this way, running at a good simmer but not boiling too hard, around 20-30 minutes or until t