Parmesan Onion Bake

Makes 6 to 8 servings.

6 medium onions, sliced
1 cup diced celery
8 tablespoons butter or margarine (divided; see note)
1/4 cup all-purpose flour
1 teaspoon salt
1/8 teaspoon pepper
1 1/2 cups milk
1/3 cup grated parmesan cheese
1/2 cup chopped pecans

In a large skillet, saut