In a large saucepan, cook butter, onion, and carrots over medium heat until onion softens (about 3 minutes). Add broccoli, chicken broth, thyme, garlic powder, salt, and pepper. Simmer until broccoli is tender, about 8 minutes.
Remove 1 cup of vegetables, coarsely chop, and set aside. Using an immersion blender, blend remaining vegetables and broth.
In a small bowl, whisk together the all-purpose flour and a small amount of the blended soup until a smooth paste forms. Gradually whisk in the remaining blended soup. Return the mixture to the saucepan and cook over medium heat, whisking constantly, until thickened and bubbly, about 3-4 minutes.
Remove from heat, stir in shredded cheese and reserved chopped vegetables, and serve immediately.