In medium sauté pan, over medium-high heat, add vegetable oil. Once hot, add onions and allow to cook for 2 minutes. Add 1 tbsp. butter, reduce to medium-low and cook, stirring frequently, until onions are caramelized and soft. Season with salt and pepper.
Combine ketchup with chipotle peppers. (Heat level can be adjusted to your preference).
To build sandwich, spread chipotle ketchup on each bread half. Add sausage patties followed by caramelized onions and cheese. Top with remaining bread slices.
In nonstick pan, over medium heat, add remaining 1 tbsp. butter. Once butter is almost melted, add sandwiches and cook on both sides until well browned and cheese is melted.