In a 6-qt. stockpot, heat 3 tablespoons butter over medium-high heat. Add mushrooms; cook and stir until tender, 8-10 minutes. Remove from pot., In same pot, heat remaining butter over medium heat. Stir in flour, salt and pepper until smooth; gradually whisk in cream and milk. Bring to a boil, stirring constantly; cook and stir until thickened, 2-3 minutes., Stir in haddock, shrimp, peas and sautéed mushrooms; cook until fish just begins to flake easily with a fork and shrimp turn pink, 5-7 minutes. Add cheese, crab and pimientos; stir gently until cheese is melted. If desired, sprinkle servings with paprika.