Spread 1 tbls salad dressing on each tortilla, covering entire surface. Top each with lettuce leaf and 2 oz of roast beef, a slice of cheese, and 1 tsp onion. Roll up each tortilla tightly. To serve, cut roll-ups into 1" slices. Insert fancy toothpick into each to secure. Can be made ahead, covered, and refrigerated up to 1 hour.