Defrost spinach either in the microwave or in hot water. Drain plain spinach well in colander and squeeze out excess water.
In saucepan, melt butter, and sauté the onion. Blend in the flour and add the milk slowly, blending well. Add the American cheese (broken in small pieces) and parmesan cheese. Cook over low heat until thickened. Spray a 2 ½ or 3-quart casserole with baking spray. Add the spinach to the casserole dish. Add the cheese sauce, but not too much to make it runny. Poke the cheddar cheese sticks down into the spinach at various places. Smooth the top and sprinkle with breadcrumbs.