Start by placing your ground chicken, egg, breadcrumbs, diced veggies, and spices into a large bowl. Gently mix everything together with your hands or a spoon until just combined. Be careful not to overwork the meat or the patties might become tough.
Divide your seasoned meat mixture into four equal portions. Roll each portion gently between your palms to form a smooth, round ball. These balls are what you will place directly into the hot pan to smash later.
Place your skillet over medium-high heat and add the avocado oil, letting it get shimmering hot. You want the pan hot enough that the meat sizzles immediately upon contact. This high heat is the secret to creating that delicious, caramelized crust.
Place the four chicken balls into the hot skillet giving them plenty of space. Immediately take your spatula and press down hard on each ball to flatten it into a thin patty. Cook these for about 4 to 5 minutes on the first side to develop a sear.
Flip the patties over; you should see a nice golden-brown crust on the cooked side. Immediately place a slice of cheddar cheese on each patty to start melting. Cover the pan with a lid and cook for another 3 to 4 minutes until the cheese is gooey.
While the chicken finishes cooking, grab your toasted brioche buns. Smear a generous amount of the mayonnaise and Sriracha mixture on both the top and bottom buns. This creates a moisture barrier and ensures every bite is full of spicy, creamy flavor.
Stack two cheesy chicken patties onto the bottom bun for a double-decker experience. Top the patties with your crispy bacon strips and fresh red onion slices. Place the top bun on the stack and serve immediately while hot and juicy.