Preheat the oven to 350 degrees. Mix the cracker crumbs with the melted butter. Press the mixture into the bottom of an 8-inch pie plate. Heat 2 tablespoons of the butter in a skillet over medium heat. Add the onions and cook, stirring, until they soften. Spoon the onions into the piecrust. Combine the eggs, milk, salt and pepper in a bowl and whisk to blend. Pour the mixture over the onions. Sprinkle with the cheese and bake until the center sets, about 30 minutes.