In four batches, place spinach in a steamer basket; place in a large saucepan over 1 in. of water. Bring to a boil; cover and steam for 3-4 minutes or until wilted. Transfer to a large bowl; keep warm.
In a large skillet, saute onions in butter until tender. Stir in the flour, salt and pepper until blended. Gradually add milk and cream. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in cheese. Drain spinach; add sauce and toss to coat.