Heat oil in a large skillet over medium heat. Add the onion and cook until slightly soft and translucent, about 5 minutes.
Add in the mushrooms, soy sauce and black pepper and cook an additional 8-10 minutes until mushrooms are browned and reduced.
Remove from heat and place in the base of a food processor. Add the beans and pulse a few times to gently break them up, then add in the remaining slider ingredients: quinoa, chili powder, cumin, smoked paprika and walnuts.
Pulse together until a mixture forms. It should be easy to ball together, but still have some texture.
Place in the fridge for at least 15 minutes; this will make for easier shaping.
Preheat oven to 375°F. Lightly spray a baking sheet with cooking oil and set aside.
Remove mixture from fridge and scoop out a heaping 3 tablespoon-size ball. Form into a patty-shape, then place on the prepared baking sheet. Repeat with the rest of the mixture.
Lightly spray the tops of the patties, then place in the oven and cook for 40 minutes, flipping halfway through, until lightly crispy and cooked through.
Serve on toasted slider buns with toppings of choice.