Cook asparagus, covered, in a small amount of boiling salted water 6 minutes or until crisp-tender; drain well. Arrange asparagus evenly in a 1 1/2-quart casserole; set aside.
Melt butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Beat egg yolks; combine with milk, stirring well. Gradually add milk mixture to flour mixture, stirring constantly. Cook over medium heat, stirring constantly, until sauce is thickened and bubbly. Add cheese, salt, and pepper; stir until cheese melts. Pour cheese sauce over asparagus.
Combine egg whites (at room temperature) and cream of tartar; beat until stiff peaks form. Spread meringue over cheese sauce, sealing to edge of casserole. Bake at 400° for 8 to 10 minutes or until meringue is lightly browned. Serve immediately.