Preheat broiler. Melt 2 tablespoons butter in heavy large skillet over medium-high heat. Add mushrooms; saute until golden, about 7 minutes. Add Cognac; simmer until liquid reduces to glaze. Reduce heat to medium. Add tomatoes, cream and thyme; simmer until sauce thickens, stirring occasionally, about 7 minutes. Season with salt and pepper. Melt remaining 2 tablespoons butter in another heavy large skillet over medium-high heat. Season fish with salt and pepper. Add fish to skillet; saute until just cooked through, about 3 minutes per side. Transfer fish to broiler-proof baking dish. Spoon sauce over. Sprinkle cheese over. Broil until cheese melts, about 2 minutes.