Steam spinach until wilted, drain and chop. Melt 2 tablespoons of butter in large skillet and saute onions until golden. Remove onions with slotted spoon, mix with spinach and place in buttered casserole. Add 1 tablespoon butter to skillet. Brown chicken and remove to warm plate. Saute mushrooms in remaining butter and remove to plate with chicken. Add wine to pan and then stir in flour. Slowly add sour cream and stir until hot and thickened. Add chicken, mushrooms, and garlic powder. Place on spinach, sprinkle with cheese and bake uncovered at 350 for 20 to 30 minutes.