Preheat oven to 350°. Without separating rolls, cut each package of rolls in half horizontally; arrange bottom halves in a greased 13x9-in. baking pan. Layer with ham, cheese and pickles; replace top halves of rolls. , In a microwave, melt butter; stir in onion and mustard. Drizzle over rolls. Bake, covered, 10 minutes. Uncover; bake until golden brown and heated through, 5-10 minutes longer.