Bring water and quinoa to a boil in a saucepan. Reduce heat to medium-low, cover, and simmer until quinoa is soft, 15 to 20 minutes.
Heat olive oil in a large skillet over medium heat. Add onions and salt; cook and stir until tender, 5 to 8 minutes.
Place sweet potatoes on a microwave-safe plate; prick all over with a fork. Cook on high, turning once, until tender, 8 to 10 minutes. Cool until easily handled, about 5 minutes.
Peel sweet potatoes and place flesh in a bowl. Add eggs, tahini, kefir, flax seeds, Parmesan cheese, sesame seeds, sunflower seeds, pumpkin seeds, soy sauce, vegetable seasoning, vinegar, salt, and pepper; beat with an electric mixer until well blended. Stir in quinoa, onions, and corn.
Stir enough spelt flour into the sweet potato mixture until it barely holds together.
Melt 1/4 teaspoon butter on a griddle over medium heat. Spoon a burger-sized patty onto the hot griddle and cook until browned, about 4 minutes. Flip; add 1 slice Cheddar cheese. Cover patty with a frying pan and cook until cheese is melted, about 4 minutes more. Repeat with remaining butter, sweet potato mixture, and Cheddar cheese.