Finely chop the onion and mince the garlic.

Melt butter in a deep saucepan over medium heat.

Saute the onion and garlic in the melted butter for 3 to 4 minutes until softened.

Add the catsup, dry sherry, light brown sugar, dry mustard, lemon juice, vinegar, Worcestershire sauce, and 1/2 cup water to the saucepan.

Bring the mixture to a boil, stirring occasionally.

Reduce heat to low and simmer uncovered for 1 hour, stirring frequently to prevent scorching.

Strain the sauce through a fine sieve to remove solids.

Let cool before serving or storing.