In a large mixing bowl, combine the ground beef, cooked quinoa, diced onions, diced bell peppers, salt, and pepper. Mix until well combined.
Form the mixture into 4 equal patties. Heat olive oil in a skillet over medium-high heat and cook the patties for about 5-6 minutes on each side, or until they reach your desired doneness.
During the last minute of cooking, place a slice of cheddar cheese on each patty to melt. Toast the whole wheat burger buns in the skillet until golden brown. Assemble the burgers by spreading mayonnaise and Dijon mustard on the buns, adding the beef patty, avocado slices, and baby spinach.