In a medium pot, cook the bacon over medium-low heat until crispy, 8 to 10 minutes. Remove to a plate lined with a paper towel to drain, then eat the bacon as a snack.
Add the onions to the pot with the bacon grease and cook over medium-low heat until the onions start to caramelize, 6 to 8 minutes. Add the garlic and cook another 2 minutes. Add the tomato paste and cook, stirring occasionally, another 3 minutes. Whisk in the stock, sugar, vinegar, cumin, cayenne, salt and pepper and bring to a simmer. Cook until thickened, 15 to 20 minutes.
Let cool to room temperature before using. This sauce is great on brisket or beef ribs.