In a small bowl, combine all ketchup ingredients together. Cover and refrigerate.
In a separate small bowl, mix vinegar and sugar until sugar is dissolved. Add sliced onion and set aside to pickle.
In a large bowl, combine the beef, salt, pepper, cheddar, and minced onion, using your hands.
Mix well. Mold 4 patties using your hands; do not press patties too tightly together, as you want moist, juicy burgers.
Add the tablespoon of canola oil to a nonstick skillet over medium-high heat. Cook 4 patties 3 to 4 minutes each side. While burgers are cooking, toast the burger buns in a toaster or under the broiler about 30 seconds to 1 minute, until golden.
STOP! WHILE BURGERS COOK, CLEAN BOWLS.
Divide spinach among 4 bun bottoms, top each with a burger patty, pickled onion and 1 tablespoon of soy ketchup and the bun top.