Heat oil in a large saucepan over medium heat. Add onions and jalapenos and cook over medium heat until onions are soft and transparent, about 5 minutes. Add salt, dry mustard, black pepper, cayenne, and garlic salt. Stir to combine and then mix in the brown sugar. Add the cider vinegar, ketchup, molasses, Worcestershire, and water and whisk all together until thoroughly combined. Reduce heat to low and simmer for about 20 to 35 minutes, whisking every few minutes to prevent sauce from burning and sticking to bottom. Strain sauce to remove onions and jalapenos before serving.