Cream of Broccoli Soup

4 cups chicken broth [1 L]
1 can cream of celery soup
1 package of frozen broccoli florets
½ cup margarine [112 g]
1 onion, chopped
¼ cup all-purpose flour [65 g]
2 cups milk [500 ml]
Salt and pepper
1 cup shredded sharp cheddar cheese [70 g]
Mix broth and soup in large pot. Bring to a boil. Add broccoli and return to a boil. Simmer for 5 minutes. Heat butter in a skillet. Add onion and saute until tender. Add flour and cook for 1 minute longer Gradually stir milk into skillet. Cook, stirring continually until it boils and thickens. Add salt and pepper to taste and mix well. Stir milk mixture into broccoli mixtures. Simmer for 10 minutes. Add cheese and continue to cook until cheese is melted.