In a medium sized pot, cover the lentils with 1 cup of water, place over high heat, and bring to a boil. Reduce the heat to a simmer and cook until the lentils are tender and water is absorbed, about 25 minutes. Set aside.
In another medium sized pot, add millet and cover with 1½ cups of water. Place over high heat and bring the water to a boil. Lower heat, and simmer until millet is cooked through and all water is absorbed, 25-30 minutes.
While the millet and lentils cook, preheat oven to 375°F. Arrange the diced sweet potatoes on a large sheet pan and drizzle with 2 teaspoons olive oil, then season with ½ teaspoon salt and ¼ teaspoon pepper. Place the sheet pan into the oven and roast until the sweet potatoes are fork tender, 25-30 minutes.
Place the millet into a food processor and blend until most of it is broken down but it still has some texture, about 10 seconds. Place millet into a large bowl and repeat the same steps with the lentils and then the sweet potatoes.
Add the rest of the ingredients to the bowl and mix until everything is combined. It's easiest to use your hands. Form the mixture into 8 patties (about ½ cup each). The burgers can be cooked immediately or chilled in the refrigerator for up to 4 days. The longer you refrigerate the burgers, the firmer they will be.
Spray a large skillet with cooking spray and place it over medium heat.Cook the burgers until browned and warmed through, 3-5 minutes on each side. Enjoy burgers with your favorite toppings.
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