1. Heat vegetable oil in a small skillet over medium-high. Add onion, garlic, and oregano and saute until tender, about 3 minutes. Place the mixture in a large bowl and cool in the refrigerator for 10 minutes.
2. Combine ketchup, Worcestershire sauce, Dijon mustard, salt, pepper, and ground meat with the cooled onion mixture and mix with your hands until well blended. With dampened hands, form the meat into 8 small patties. Tip: If the burgers aren't holding together well, partially freeze them (up to 1 hour) before grilling.
3. Grill the burgers on oiled racks to desired doneness and top with the cheese of your choice. Slice potato rolls and toast the cut sides for a minute or two.
4. To make the chipotle mayo, combine mayonnaise and chipotle chile. Serve the burgers with the toppings of your choice.