Broccoli Potato Soup

1/4 c butter
1 onion chopped
1/2 c all purpose flour
2 cartons chicken broth (32 oz)
2 c milk
2 lbs red potatoes cut into 1/2 in pieces
2 c chopped broccoli florets
1 tsp dried thyme
1 /2 tsp salt
1/2 tsp pepper
1 package shredded cheddar cheese
1/2 c sour cream

In large dutch oven, melt butter over med heat. Add onion and cook stirring frequently for 5 mins. Add flour, cook, for 2 mins. Gradually stir in broth and milk, stirring until smooth. Stir in potatoes and next 4 ingredients. Bring to a boil over medium heat, reduce heat and simmer. Simmer for 15 to 20 mins or until mixture thickens and potatoes are tender. Add cheese and sour cream stirring until cheese melts.