Boil water and chicken base. Add broccoli and cook until almost done. While broccoli is cooking, make the white sauce: Melt butter in a pan. Add flour and dry mustard and mix well. Whisk in milk and add pepper to taste. Add white sauce and sautéed onions to the broccoli mixture. Stir and add cheese until melted.
I usually add more broccoli and sometimes use cheddar cheese instead of Swiss.