In a large saucepan, sauté onion in butter until tender. Stir in flour until blended. Gradually stir in milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat; stir in cheese sauce until smooth. Remove from heat.
Cook Broccoli Cauliflower blend according to package directions; drain and place in a bowl. Add the cheese sauce mixture, eggs, 1/4 cup cracker crumbs, salt and pepper.
Transfer to a greased 1-1/2-qt. baking dish; sprinkle with remaining cracker crumbs. Place dish in a larger baking pan. Fill pan with hot water to a depth of 1 in. Bake, uncovered, at 350° for 50 minutes or until a thermometer reads 160°.
