Combine vegetables, onion, garlic, and broth in a 3-quart saucepan; cook for 2 minutes. Add chicken; cook for 7 to 8 minutes, until cooked through. Whisk cornstarch into milk; add to saucepan. Cook and stir until thickened and bubbly. Meanwhile, stack the phyllo sheets, coating each with nonstick spray. Use a round cutter to cut phyllo the same size as the top of the ramekins.
Divide the mixture among eight ramekins. Top with phyllo rounds and bake 15 minutes or until golden.