Sauté the mushrooms, kale and garlic with broth or oil over medium heat until the mushrooms are soft and the liquid has evaporated, about 10 to 15 minutes. Set aside.
Place the the lentils, bread crumbs, oats, walnuts, sauce, thyme, basil, oregano and salt in a food processor and pulse until well chopped. You want a crumblelike texture. Check to make sure the mixture sticks together, and if it doesn’t, pulse some more, but make sure it doesn’t turn into a purée. You can also add a bit more pizza sauce if it doesn’t stick yet, however, you don’t want it to be too wet.
Place the mushrooms, kale and garlic into the processor, making sure not to add any leftover juices, and pulse a few times until everything is well chopped and incorporated. If the ingredients aren’t mixed after a few pulses, put the mixture in a bowl and mix it with your hands.
Place the mixture into a bowl, and put it in the fridge for at least 20 minutes; an hour is best. Making this the day before and letting it sit overnight is optimal. This helps all flavors marinate and leads to a sturdier burger.
To cook, shape the mixture into patties. If you’re not using a nonstick pan, then you may want to prepare the pan by brushing it with a tad of oil to avoid sticking. Place the patties in a pan on the stovetop over medium heat for 15 minutes on each side. Pans and stovetops vary, so begin to check the patties at 10 minutes. If you try to slide yourspatula under a patty and it won’t give, then it isn’t ready and needs to cook longer.
You can also bake the patties on a parchment-lined cookie sheet at 375F (191C) for about 20 to 25 minutes on each side. Allow the patties to cool and set for 5 to 7 minutes. Serve on buns with the toppings of your choice.