Saute onion in butter until translucent. Add flour and cook, stirring constantly for 2 minutes. Add chicken stock, bring to a simmer. Add broccoli and then add Velveeta one handful at a time, stirring until melted. Once all of the Velveeta is melted in, begin adding cheddar one handful at a time, stirring until melted. Turn off heat and slowly add beer. It foams a lot, stir constantly. Serve with crusty bread for dipping.