Heat the extra virgin olive oil in a large skillet set over medium heat. Add the mushrooms along with a pinch of salt and cook, stirring occasionally, until they release their moisture and begin to brown, 6-8 minutes.
2 tablespoons extra virgin olive oil, 6 ounces button mushrooms
Add the onion and cook until softened, about 5-6 minutes. If the bottom of the pan starts to brown, add a splash of water and stir the brown bits back into the vegetables. Stir in the garlic and cook for 30-60 seconds until fragrant.
3/4 cup finely diced yellow onion, 2 cloves garlic
Add the smoked paprika, cumin, salt, and black pepper. Stir in 1 tablespoon tomato paste and cook for 1-2 minutes, stirring frequently. Stir in the soy sauce, scrape up any brown bits, then remove from the heat and let cool slightly.
In a large bowl, mash the black beans with a fork or potato masher until mostly broken down but still textured.
15 ounces canned black beans
Add the mushroom mixture, panko, flax egg, 2 teaspoons tomato paste, brown sugar, and mustard. Mix until fully combined. The mixture should hold together when pressed; if too wet, add a bit more panko.
1 cup panko breadcrumbs, 2 tablespoons ground flaxseed mixed with 5 tablespoons water, 2 teaspoons tomato paste, 1/2 teaspoon brown sugar, 1 teaspoon Dijon mustard
Form into 4-6 equal patties and place on a parchment-lined tray. Chill for at least 20-30 minutes to help them firm up.
Heat a thin layer of oil in a skillet over medium heat. Cook the patties for 4-5 minutes per side until browned and heated through. Reduce the heat to medium-low if the burgers are browning too quickly.
Serve on toasted buns with toppings of your choice.
Notes
You can 8 ounces of mushrooms instead of 6 ounces if you'd like to use an entire container.
For a smaller burger, divide mixture into 6 patties instead of 4, making sure to flatten to fit the size of the bun.
To grill, oil the grates well and grill over medium-low, adjusting heat as needed.