In small bowl, beat egg and water to form egg wash. Brush top of each biscuit with egg wash; coat top with crushed pretzels. Bake as directed on can. Cool. Split biscuits; set aside.
In 10-inch skillet, brown onion rings over medium heat; set aside.
In same skillet, brown meatballs until golden. With back of spoon, gently press down on meatballs to form a burger shape. Remove from heat. Top each with slice of cheese; let stand until melted.
To assemble each slider, top bottom half of biscuit with burger, onion ring and 1 tablespoon steak sauce; cover with top half of biscuit.