1. Heat the oil in a skillet over medium heat. Add the shallots and cook until they begin to soften, about 2 minutes.
2. Turn the heat to medium-high, add the mushrooms and sauté 2 minutes more.
3. Add the tomatoes, tarragon, salt and pepper. Heat thoroughly. If using canned tomatoes, simmer for 30 minutes to reduce the acidic taste.
4. Adjust the salt and pepper to taste. (This can be made ahead and kept in the refrigerator for up to 2 days.)
For the Sautéed Sea Bass:
1. Generously season the fillets with salt and pepper.
2. Heat the oil over medium-high heat in a pan large enough to accomodate all the fillets.
3. Sear the fillets for 3 to 4 minutes on each side, turning only once. (Thin fillets will take less time, thicker fillets more.)
4. Serve the fish with the mushroom ragout spooned over and around the fish.