In a large pot over medium-high heat, melt butter. Add onions, carrots and broccoli. Stir. Cook for about 5 minutes, or until tender. 

To the pot, slowly add chicken broth, milk, heavy cream and flour. Stir to combine. 

Sprinkle in salt, pepper and chili flakes. Stir and cook for about 10 to 15 minutes, or until soup thickens. 

Once soup is thick, stir in cheddar cheese and heat until the cheese is completely melted. 

Remove from heat immediately after cheese is mixed in and serve.