Preheat oven to 375°. Remove stems from mushrooms and finely chop stems; set caps aside. In a large skillet, melt 2 tablespoons butter over medium heat. Add chopped stems and onion; cook and stir until tender, 4-6 minutes., Add cream cheese and seasoned salt; cook and stir until cream cheese is melted. Fill mushroom caps. Melt remaining 2 tablespoons butter. Dip bottoms of mushrooms in melted butter; place on a baking sheet. Top with cheddar cheese. For topping, combine crackers and butter; sprinkle over tops., Bake until mushrooms are tender and heated through, 12-15 minutes. Serve hot.