In a saucepan, melt butter; stir in flour and salt until smooth. Add milk gradually and continue cooking, stirring constantly, until sauce is slightly thickened. Add shredded cheese to the sauce.
Combine cracker crumbs and chopped walnuts. Into a buttered casserole, place a layer of asparagus, a layer of cracker mixture and a layer of the sauce. Repeat layers, using remaining ingredients. Bake at 375° oven until top is nicely browned, about 15 to 20 minutes.
Calories per serving: 355