Under high heat melt butter & cook onions and juice until clear
Add garlic and cook for 30 seconds
Slowly add flour and cook for about 5 minutes (mixture should be very thick with no visible butter)
Slowly add broth and sherry (consistence of mixture should be like whipped potatoes)
Slowly add ½ & ½ and bring to slow rolling boil being sure to stir constantly
Reduce heat to medium and stir constantly until desired thickness is achieved
Add crab
Add 1 TBL Spoon Butter to finish
Salt and pepper to taste
Some people will want to add more Sherry in their individual bowls
Garnish with Old Bay
The key is to cook the flour before adding the chicken stock plus constant stirring is required during the whole process.