Heat oven to 400°F. Spray cookie sheet with cooking spray. In small bowl, stir mustard and mayonnaise until smooth; set aside.
Place biscuits on microwavable plate. Microwave uncovered on Medium (50%) 30 to 45 seconds, turning over halfway through microwave time. (Biscuits should still be cold for easier handling.) On cookie sheet, press and stretch each biscuit into 5-inch round. Bake biscuits 6 to 8 minutes or until golden brown.
Meanwhile, in 6- to 8-inch nonstick skillet, melt butter over medium heat. Pour eggs into skillet; beat. Sprinkle with ham. As mixture begins to set at bottom and side, gently lift cooked portions with spatula so that thin, uncooked portions can flow to bottom. Avoid constant stirring. Cook 3 to 4 minutes or until eggs are thickened throughout but still moist.
Spread mustard mixture on each biscuit. Top each with egg mixture. Sprinkle with onion and cheeses. Bake 3 to 4 minutes or until hot and cheese is melted.