Mix together the mayonnaise, Sriracha and maple syrup in a small bowl. Spread on the inside of both bun halves. Stack the lettuce, tomato and onion on the bottom bun.
Pat and squeeze the sausage together into a ball. Put it on a piece of plastic wrap, and flatten it out into a thin patty 4 to 5 inches wide.
Heat the oil in a small nonstick skillet over medium-high heat. Remove the patty from the plastic wrap, and cook it until browned on both sides and cooked through, about 3 minutes per side. Lay the Cheddar on the patty, cover the skillet and cook until the Cheddar melts, about 30 seconds. Remove, and set the patty on the bottom bun.
Crack the egg into the skillet over medium-high heat, sprinkle with some salt and pepper and cook until the egg whites are set and crispy on the bottom and edges and the yolk is still runny, 2 to 3 minutes. Set the egg on top of the sausage patty, and top with the other bun half. Eat right away.
If you can't find bulk breakfast sausage, remove the casing from breakfast sausage links and form into 1 large patty.
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