Cut a 1/4-inch slice from the top of each onion. Reserve slices for use in another recipe.
Cover and cook onions in boiling salted water 30 minutes or until tender but not mushy. Drain, and place in a lightly greased 13- x 9- x 2-inch baking dish; set aside.
Melt 1/4 cup butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Add salt and cheese, stirring until cheese melts. Pour cheese sauce over onions.
Melt remaining butter; toss with breadcrumbs. Sprinkle buttered breadcrumbs and toasted almonds over onions. Bake at 350° for 15 minutes or until lightly browned.