In a heated skillet, add 1 tablespoon of olive oil and diced onions. Add a pinch or two of salt and cook over medium-low heat until caramelized, approximately 15 minutes. Remove from heat and let cool. Set aside 1/4 cup of the onions for topping.

In a large bowl, add ground turkey, mushrooms, Panko breadcrumbs, Parmesan, dry onion soup mix, minced garlic, remaining olive oil, Worcestershire sauce, and cooled caramelized onions. Fold gently until just incorporated. Shape into 4 patties, approximately 6 oz / 170g each. Place on a parchment-lined sheet pan and refrigerate for at least 20 minutes.

Preheat your grill or grill pan to 400°F / 200°C (medium-high heat). Alternatively, preheat your oven to 400°F / 200°C (180°C fan-forced) and line a sheet pan with parchment paper.

Remove burger patties from the refrigerator. Sprinkle patties with salt and pepper before grilling or placing them in the oven.

If using the grill, place burger patties on the grill and close the lid. Cook for 5-6 minutes per side, until the internal temperature reaches 165°F / 74°C. If using the oven, place patties on the prepared sheet pan and bake for 7-8 minutes per side, 14-16 minutes total, until the internal temperature reaches 165°F / 74°C.

During the last 1-2 minutes of cooking, top each patty with a slice of Swiss cheese and close the grill lid (or return the pan to the oven) until the cheese is melted. Transfer to hamburger buns and top with the reserved caramelized onions. Serve immediately.