Heat oven to 350°F.
Arrange crackers in a single layer on a baking sheet flat side down. Place a lightly folded slice of cheese on each RITZ Cracker.
Lay a lightly folded slice of ham on top of the cheese. Top with another lightly folded slice of cheese.
Finish each mini sandwich with another RITZ Cracker, rounded side up.
Mix melted butter, Dijon mustard, Worcestershire sauce, and salt. Pour the butter mixture evenly over the mini sandwiches.
Sprinkle the poppy seeds and the dried onion flakes evenly over the top.
Bake for 5–8 minutes, or until the cheese is melted and the tops of the Ritz Crackers are lightly golden brown. Serve warm.