1. To make the sauce, heat oil in a large skillet over medium-high heat. Add fennel and saute for 5 to 7 minutes or until softened and lightly golden. Add mushrooms and cook until softened, about 3 minutes. Add tomatoes and increase heat to high. Cover and cook until the tomatoes begin to fall apart and thicken, 4 to 5 minutes. Remove from heat and reserve.
2. To poach the salmon, put it in a deep skillet just big enough to hold it. Add the vermouth, then enough water to cover the salmon. Place on medium heat until it begins to simmer. Poach for 10 minutes per inch at the thickest point, usually 7-10 minutes, depending on how thick the fish is.
3. Just before serving, add olives, seasoning salt to taste and 1/4 cup of the poaching liquid to the sauce. Bring to a simmer over medium-high heat and reduce the mixture for a minute until slightly thickened. Transfer the fish pieces to a platter or individual serving plates. Blot any excess liquid and spoon the sauce over each piece. Serve immediately.
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