Cook spinach according to directions on package. Drain and reserve liquor. Melt butter in saucepan over low heat. Add flour, stirring until blended and smooth, but not brown. Add onion and cook until soft but not brown. Add liquid slowly, stirring constantly to avoid lumps. Cook until smooth and thick; continue stirring. Add seasoning, jalapenos, and cheese which has been cut into small pieces. Stir until melted. Combine with cooked spinach. This may be served immediately or put into a casserole and topped with buttered bread crumbs. The flavor is improved if the latter is done and kept in refrigerator overnight. This may also be frozen.