Coarse kosher salt and freshly ground black pepper
Heat 1 teaspoon olive oil in a large pan over medium heat. Add garlic and cook, stirring for 30 seconds or until garlic is fragrant. Add the mushrooms and onions and continue to cook, stirring occasionally, until softened, 5-8 minutes.
Place walnuts in a food processor and pulse until powdery but not sticky. Add the mushroom mixture and process until well combined and there are no recognizable ingredients. If the mushrooms are still hot I usually open the feeding tube on the processor a bit to let steam out while I process.
Transfer the mixture to a large bowl. Add the breadcrumbs, egg, and season with salt and pepper. Stir until the mixture is well combined. Cover with plastic wrap and let rest in the refrigerator for at least 30 minutes. You can also prepare this mixture in the morning and let rest in the refrigerator until supper time.
Form the mixture into four thin patties. If you make them thick it will be hard to get them to cook through before the outside is done.
In a cast iron or non-stick pan, heat 2-3 tablespoons olive oil over medium heat. Once hot, add the patties and allow to cook 2-3 minutes per side until a deep brown color. Transfer to a paper towel lined plate and serve on buns with all your favorite burger fixings.