Reduce oven setting to 350F. Coat 36 mini muffin cups (1-3/4 x 3/4-inch) generously with cooking spray.
Beat eggs, milk, salt and pepper in medium bowl until blended. Add cheese, zucchini, bell pepper and onion; mix well. SPOON about 1 Tbsp. egg mixture into each muffin cup.
Bake in 350F oven until just set, 10 to 13 minutes. CooL on wire rack 5 minutes; remove from cups.
To serve, Spread about 1 tsp. cream cheese on each crostini; top with a mini frittata.