Preheat your oven to 350°F. Arrange 24 Ritz crackers, flat side down, in a 9x13-inch baking dish. Tear the deli ham into strips and fold a piece onto each cracker. Cut the Swiss cheese into roughly 1-inch squares and set a slice atop the ham-covered crackers. Cover each with a second cracker to create small sandwiches. In a small bowl, combine the melted butter with Dijon mustard, Worcestershire sauce, and garlic powder, whisking until uniform. Drizzle this mixture evenly over the sandwiches. Finish by sprinkling poppy seeds and minced onion across the tops. Bake for 10–12 minutes, or until the cheese is melted and the cracker tops are golden. Serve warm.