Heat the vegetable oil in a medium saucepan over medium heat until shimmering, 1–2 minutes.
Add the yellow onion and cook, stirring occasionally, until translucent and lightly golden, 6–8 minutes.
Stir in the garlic and cook until fragrant, 30–45 seconds.
Add the ketchup, water, apple cider vinegar, yellow mustard, Worcestershire sauce, chili powder, paprika, ground cumin, cayenne pepper, dark brown sugar, molasses, kosher salt, and black pepper. Whisk to combine and bring to a gentle simmer over medium heat, 3–5 minutes.
Reduce heat to low and simmer uncovered, stirring occasionally, until slightly thickened and glossy, 20–30 minutes; the sauce should coat the back of a spoon and yield about 3 cups.
Taste and adjust seasoning: add a pinch of kosher salt for savoriness, a splash of apple cider vinegar for tang, or a teaspoon of dark brown sugar for balance as needed.
Let cool 10 minutes. Use warm for brushing during the last 5–10 minutes of grilling or as a table sauce. Refrigerate in a sealed jar up to 2 weeks.