Coarsely dice onion into pieces roughly half aninch in size. Set aside.
In a food processor, pulse mushrooms and half of the diced onions until the pieces are roughly the size of rice grains.
After pulsing the mushrooms and onions, sauté the mixture in 1 tsp. oil for about 4-7 minutes, or until the mushrooms lightly caramelize and the onion turns translucent. Allow the mushroom mixture to cool down for a few minutes or until semi-cool to the touch. Once cooled, transfer to a large bowl.
In large bowl, add ground beef, egg, andseasonings to the mixture, and mix until all of the ingredients are wellcombined. Afterwards, form the mixture into 4-6 patties, and then sauté them oneach side for about 3-5 minutes.
Immediately top each patty with a slice of Swiss cheese, and then serve assemble the burger with your favorite toppings. Enjoy!
Recipe is 421 calories with the patty and Swiss cheese.