Melt butter in a Dutch oven over medium heat; add onion, and cook, stirring often, 3 to 5 minutes or until tender. Sprinkle onion mixture with flour, and cook, stirring constantly, 1 minute. Gradually whisk in chicken broth, salt, and pepper. Add potatoes, and bring to a boil, stirring often. Reduce heat to medium-low; simmer, stirring occasionally, 15 minutes or until potatoes are tender. Whisk together heavy cream and sour cream in a bowl. Gradually stir about 1 cup hot potato mixture into cream mixture; add cream mixture to remaining hot potato mixture, stirring constantly. Stir in 1 cup Cheddar cheese and thawed broccoli florets. Cook 5 minutes or until thoroughly heated. Top with chopped cooked bacon, sliced green onions, and shredded Cheddar cheese.