In a Dutch oven, melt butter over medium low heat. Add onions and cook, covered, until tender. Stir in flour until blended; cook 1 minute, stirring constantly.
Gradually stir in milk, soup base, and pepper; cook, stirring constantly, until thickened. Stir in cream. Reheat just until broth is boiling. Add salt if desired. Combine with potatoes and pour into a greased 9x13" baking dish.
Bake, covered tightly with foil, at 375 for 45 minutes-1 hour.
Remove foil and bake on speed bake for another 45 minutes, until done. Note: If you don't have speed bake, increase the oven temperature to 400. Add cheese to top, if desired, during the final 15 minutes of baking time.