Heat the oven to 375 degrees F. Spray 16 (2 1/2-inch) muffin-pan cups with the cooking spray. Stir the green beans, soup, milk, 1 cup cheese and 2/3 cup onions in a large bowl.
Roll each biscuit into a 4-inch circle. Press the biscuit circles into the bottoms and up the sides of the muffin-pan cups. Divide the green bean mixture among the biscuit cups.
Bake for 20 minutes or until the biscuit cups are golden brown. Stir the remaining cheese and onions in a small bowl. Sprinkle over the green bean mixture.
Bake for 5 minutes or until the cheese is melted. Let the minis cool in the pans on wire racks for 5 minutes.