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.