Preheat oven to 400°F. Bake the sweet potato for 50-60 minutes or until soft.
While the sweet potato is baking, cook the millet until soft, about 20-30 minutes.
Once the sweet potato is baked and cooled, combine sweet potato, oats, one cup of black beans, cilantro, garlic powder, cumin, salt, pepper, and 1 tablespoon oil in a food processor. Mix until smooth.
In a separate bowl, mix together the sweet potato mixture with the remaining beans, the corn, and millet.
Heat the remaining tablespoon of oil in a large pan on medium heat. Place heaping spoonful's of the mixture onto the hot pan, and use the back of the spoon to pat them flat and form four-inch-diameter patties. Brown both sides of each burger.
While the patties are cooking, mash the avocado with a fork. Mix in the Greek yogurt, lime juice, and salt. Stir in the tomatoes, and set aside
Serve on a bun with a dollop of avocado cream sauce.
NOTE: To make this recipe gluten-free, use certified gluten-free oats and gluten-free hamburger buns. To veganize this recipe, substitute soy yogurt for Greek yogurt when making the cream sauce.