This is a savoury barbecue sauce - if you prefer a sweet barbecue sauce I would suggest adding more brown sugar, molasses or honey.
Adapted from Serious Eats, makes 2 cups
2 tablespoons olive oil
1 medium onion, minced
3 cloves garlic, minced
1 bottle (341 mL) Guinness Beer
2 cans (398 mL) tomato sauce
1/4 cup molasses
1/4 cup apple cider vinegar
1 tablespoon dijon mustard
2 tablespoons brown sugar
2 tablespoons Worcestershire sauce
In a medium saucepan, saute the onion and garlic in the oil until soft, about 5 minutes. Add the remaining ingredients and bring to a boil. Simmer until reduced to a medium-thin thickness, about 30 to 45 minutes.
Transfer to a blender. Puree until smooth.
Store in the refrigerator for up to two weeks.