In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the finely diced onion and sauté for 5 to 6 minutes until the onion is softened and browned. Remove from the heat and let cool briefly before using.
In a mixing bowl, combine the ground beef and pork with the sautéed onions, breadcrumbs, beaten egg, all the spices, kosher salt, and pepper. Mix well using your hands. Divide the mixture into 6 or 8 portions (*see note) and shape into burger patties. The patties can be made ahead of time and frozen for later use. They can be refrigerated for a day before they should be cooked.
Maple Mustard Aioli
In a small bowl combine the mayonnaise, maple syrup, grainy and Dijon mustards, kosher salt and pepper. Whisk together and chill until ready to serve.
To Serve
Heat a skillet over medium high heat. Add the remaining tablespoon of olive oil and sauté the mushrooms for 4 to 5 minutes until browned. Remove from the heat and keep warm until ready to assemble.
Heat a grill over medium high heat. Spray the grill with non-stick spray. Place the patties on the grill and cook for 4 to 5 minutes. Wait until the edges of the patties look cooked and there are char marks on the bottom. Flip the patties and cook for another 4 to 5 minutes until fully cooked and they reach an internal temperature of 165°F.
Turn the heat down and top the burgers with the sliced cheese, heating until it melts. Toast the buns on the grill as well, if desired.
To assemble the burgers, spread the maple mustard aioli on the tops and bottoms of the buns. Layer the burgers with a leaf of lettuce, the patties, and the sautéed mushrooms. Top with a sliced pickle if desired and enjoy!
Notes
*This recipe will make either six 6 oz patties or eight 4 oz patties.