Place bacon in a large, heavy-bottomed saucepan over medium heat. Cook until fat is rendered. Add 2 tablespoons butter, 1 tablespoon oil, and leeks. Cook, stirring, until soft, 2–3 minutes. Add mushrooms and cook, stirring occasionally, until softened and moisture has evaporated, 8–10 minutes. Add onions and garlic and cook, stirring, until softened, 6–8 minutes. Stir in tomato paste and flour and cook for 1 minute, stirring continuously. Add Marsala wine and bring to a boil, scraping the bottom on the pan with a wooden to spoon to release any fond. Add chicken broth, sugar, salt, sage, rosemary, red pepper flakes, vinegar, and bay leaf. Bring to a simmer and cook until reduced slightly, about 15 minutes, stirring occasionally to prevent scorching. Adjust seasonings with additional salt, pepper, sugar, or vinegar to taste.
Meanwhile, heat oven to 400°. Heat remaining 1 tablespoon oil in a large, oven-safe nonstick or well-seasoned skillet over medium heat. Season fish with salt and pepper, then dust one side with flour. Shake off excess flour and sear fish, flour side down, in the hot oil. Once fish begins to brown, add remaining 2 tablespoons butter and transfer skillet to oven to continue cooking until cooked through, 6–8 minutes. Remove fish from oven, flip over and baste with pan juices.
Serve immediately topped with Marsala sauce and alongside mashed potatoes and sautéed spinach, if desired.