Tomato concassé is a mixture of peeled, seeded tomatoes that have been sautéed with shallots and herbs. Tomato concassé is a traditional French preparation used in many sauce and stuffing recipes.
Bring a pot of water to a boil over high heat. Set up an ice bath. Score the bottoms of the tomatoes and core the stems from the top. Drop the tomatoes in boiling water for 15 seconds (you want the skin to start pulling away but for the tomatoes not to become pastry), and transfer to the ice bath.
Peel off the skin (it should come away easily and quickly), quarter, and seed the tomatoes, straining the seeds if you’d like to return the juice to the concassé, or want to use it to flavor something else (like mushroom duxelles).
Dice the tomato flesh. Heat the oil and butter in a sauté pan over low heat. Add the shallot and garlic and simmer until translucent. Then add the chopped tomatoes, thyme, and oregano. Season with salt and pepper, to taste.
Cook until the mixture is soft and the moisture has evaporated, but taking care not to overcook, stirring occasionally, for about 15-20 minutes. Season with salt and pepper, to taste.