Credit where credit is due. This recipe is an adaptation of Ina Garten's recipe for Niman Ranch Burgers. We are biased, but we think our Otsquago Creek Grass-Fed beef tastes even better than Niman Ranch.
Place olive oil and butter in a skillet or saute pan that has a tightly fitting lid. Turn heat up to medium.
Add onions and thyme and stir to coat with oil and butter. Turn heat down to medium low. Place lid on the pan and cook onions for about 10 minutes.
Remove lid and continue to cook onions, stirring frequently, for about another 20 minutes - or until the onions are caramelized and golden brown. Depending on how hot your stove runs, you may need to turn the heat down to "low" to keep the onions from scorching while they cook.
At the end of cooking, add the vinegar, salt and pepper. Cook another 2-3 minutes. Then keep warm until you are ready to serve.
If you are using a charcoal grill, get your fire started and ready to cook over. If using a gas grill, start warming the gas grill.
Place Ground Beef, Olive Oil, Dijon Mustard, Salt and Pepper in a Bowl. Use a fork to gently but thoroughly blend the burger ingredients. (Avoid over handling the meat mixture.)
After you have mixed the burger ingredients using a fork, form the mixture into 6 burgers (about 5.5 ounces each) of equal diameter and thickness. You can use a burger press if you wish.
Have your grill at medium heat - so burgers don't get scorched.
Place burgers on the grill and cook for about 3 minutes on the first side. While the burgers are cooking, split your burger buns so they are ready to warm on the grill.
Flip the burgers and cook another 3 minutes on the 2nd side - to achieve medium rare. (You can cook longer if you like more done-ness for your burger. But keep in mind that grass-fed beef is best served medium to medium rare ... or RARE if you are me.) Once the burgers are cooked to the desired done-ness, remove to a warm platter and let rest while you toast the buns.
Place burger buns with the "inside" facing down on the grill until they are toasty.
Remove buns and spread each bun with one tablespoon of mayonaisse.
Place burger on a bun and top with caramelized onions.
This dish is, in my opinion, complete. But your guests may want additional condiments.