1. Pre-heat grill.
2. place the ground beef in a large bowl and add the mustard, olive oil, salt, and pepper.
3. mix gently with a fork to combine
4. shape the meat into 4 patties of equal size and thickness.
5. place the burgers on the grill and cook for 4 minutes.
6. turn the burgers and cook for another 3 to 4 minutes, until medium-rare
7. Top with cheese if you desire
8. break apart the english muffins and toast the halves cut side down on the grill.
9. spread each half with mayonnaise and top with a burger and then with a heaping tablespoon of caramelized onions.

caramelized onions:

1. heat the olive oil and butter in a large shallow pot, add the onions and thyme, and toss with the oil.
2. place the lid on top and cook over medium-low heat for about 10 minutes to sweat the onions.
3. remove the lid and continue to cook over medium-low heat, stirring occasionally, for 25 minutes, until the onions are caramelized and golden brown.
4. add the vinegar, salt, and pepper and cook for 2 more minutes, scraping the brown bits from the pan.