Bring the lentils and 1 3/4 cup of the broth to a boil in a medium saucepan over high heat. Reduce heat to medium-low, partially cover and simmer until the lentils are fully softened and the liquid is absorbed, about 30 minutes. Transfer to a medium bowl with the remaining 1 tablespoon broth and mash well with a potato masher. Set aside.
Heat the oil in a large nonstick skillet over medium heat. Add the chopped onion, lemon juice and 1/4 teaspoon salt and cook, stirring, until softened, about 6 minutes. Add the spinach, garlic, 1 1/2 teaspoons black pepper and cumin and stir until the spinach is wilted, about 3 minutes.
Add the spinach mixture, breadcrumbs, walnuts and 3/4 teaspoon salt to the lentils and mix thoroughly. Cover and refrigerate for at least 1 hour or overnight.
Preheat the grill to medium-high. Form the mixture into six 4-inch patties and spray with cooking spray on each side. Grill until nice grill marks form, about 3 minutes per side. Place the patties in the buns with the sliced onion and other toppings, as desired, and serve.
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