Meanwhile, in a small saucepan, bring 3/4 cup water and the quinoa to a boil. Reduce to a low simmer and cover, until the water is nearly absorbed and the quinoa appears transparent, about 8 to 10 minutes. Remove quinoa from the stove; allow to sit, covered, for 5 minutes.

In a separate saucepan, melt 1 tablespoon of the butter. Stir in 1 tablespoon of maple syrup and the cayenne pepper. Add mixture to the mashed sweetpotatoes along with the kale, cooked quinoa, pecans and 1/2 teaspoon salt; mix well.

Form the mixture into four patties; place on a well-oiled baking sheet. Bake for 35 minutes, turning once at the halfway point.

When the burgers have around 15 minutes left to bake, make the onion topping. In a small skillet over medium heat, melt the remaining 1 tablespoon butter. Add the onion; sprinkle with salt and fresh ground black pepper to taste. Cook and stir for 12 minutes. Reduce heat to low; add remaining 1 tablespoon of maple syrup. Cook and stir until onions are slightly brown, about 3 more minutes; remove from heat.

Transfer burgers to buns and top each with onion mixture.