In the meantime, melt butter in a small saucepan over medium heat. Add flour and stir quickly till it dissolves. Pour in milk and continue to stir vigourously so no clumps form, 2-3 minutes. Add in cream and sour cream. Continue whisking until mixture thickens. another 3-4 minutes (can nicely coat back of a wooden spoon). Pour milk mixture into soup. Stir and season once more with salt and pepper.
Let simmer together for amother 5-10 minutes, lid closed. Turn off heat and serve, garnishing with cheddar, bacon and scallions.