In a large skillet over medium heat add the oil and butter.

Once the butter has melted add the sliced onions and cook for 3 minutes to start to soften them.

Add the mushrooms and sauté for 4 more minutes until the onions and mushrooms are mostly softened.

Add the mustard, water, Worcestershire sauce, salt, and pepper and cook an additional 4-5 minutes to reduce the sauce and make the veggies totally softened. Remove from heat.

Preheat your grill to medium high- you are aiming for 375-400 degrees F.

In a large mixing bowl with your hands or using a wooden spoon, combine the beef, seasoning salt, Worcestershire sauce, pepper, and garlic powder until just combined.

Divide the meat into 4 equal pieces and form into patties slightly larger than the buns you are using.

Grill each side of the patties for 4-6 minutes depending on the temperature of meat you’d like for your burger. For medium-well burgers, grill until the center reaches a temperature of 150. During the last 2-3 minutes of grilling add a slice or 2 of Swiss cheese to each patty.

Assemble your burgers and top with a generous spoonful of prepared mushrooms and any desired condiments.