Split mushrooms in half and slice half of them. Then chop the other half.
Melt butter over med-low heat in a soup kettle and add onion and mushrooms. Cover and simmer on low for 7-10 minutes until onions are tender and mushrooms are golden brown.
Add flour, salt & pepper and mix for 1-2 minutes until fully combined.
Add chicken broth and bring to a boiling, stirring constantly for 1-2 minutes.