Drain spinach well, gently pressing between layers of paper towels.
Melt 1/4 cup butter in a large skillet over medium-high heat; add onion, and saute until tender. Add flour; cook, stirring constantly, 1 minute. Add milk and next 4 ingredients, stirring until cheese melts. Stir in spinach. Spoon into a lightly greased 11- x 7-inch baking dish.
Stir together cracker crumbs and 1/4 cup melted butter; sprinkle over spinach mixture.
Bake at 350° for 30 minutes or until bubbly. Let stand 5 minutes.
*6 ounces mild Mexican loaf pasteurized cheese product may be substituted for jalapeño pepper cheese.