Melt 2 tablespoons butter in a large skillet over medium heat. Cook and stir onion and bell pepper in melted butter until the onions are translucent, 5 to 7 minutes. Stir flour, salt, paprika, and mustard into the onion mixture, reduce heat to low, and continue cooking until the mixture is bubbling, 2 to 3 minutes. Gradually stream evaporated milk into the mixture while stirring; bring to a boil for 1 minute. Add corn and egg; stir. Pour into a casserole dish.
Mix crackers and 1 tablespoon melted butter together in a bowl; sprinkle evenly over corn mixture.
