1. Preheat oven to 375°.
2. Place bread cubes on a jelly-roll pan; coat with cooking spray. Bake at 375° for 10 minutes or until lightly toasted, turning once. Remove from oven; cool.
3. Combine bread, 1/2 cup cheese, 3 tablespoons onions, and next 5 ingredients (through egg yolks) in a large bowl. Place egg whites in a small bowl, and beat with a mixer at high speed until foamy (about 30 seconds). Gently fold egg whites into bread mixture.
4. Spoon about 1 cup bread mixture into each of 4 (7-ounce) ramekins coated with cooking spray. Divide the remaining 1/4 cup cheese and remaining 1 tablespoon onions evenly among ramekins. Bake at 375° for 20 minutes or until lightly browned. Top each serving with 1 teaspoon sour cream.
Wine Match by Scott Jones: These puddings need a fruity red to cut the richness and balance the sharp cheddar and salty ham--Colonia Las Liebres Bonarda 2009 (Argentine, $10) is ideal.