6 tablespoons (3/4 stick) butter, room temperature
2 pounds fresh broccoli, stems and florets separated and chopped into bite-size pieces
1 large onion, chopped
2 garlic cloves, minced
1 teaspoon chopped fresh tarragon or 1/2 teaspoon dried
6 1/2 cups chicken stock or canned low-salt chicken broth
1 cup whipping cream

3 tablespoons all purpose flour

2 cups (packed) grated extra-sharp cheddar cheese (about 8 ounces)