In a small bowl, combine mayonnaise and sriracha for spicy aioli. Mix until smooth and set aside.

Heat oil in a skillet over medium-high. Sauté half a yellow onion for about 5 minutes, then lower heat to medium-low and cook for another 20 minutes until golden brown.

In another skillet, heat oil and cook sausage patties for 5-7 minutes each side. In the last minute, add cheese on top to melt.

Whisk eggs and chives in a bowl. In a nonstick skillet, heat oil and pour in the egg mixture. Cook gently for 3-4 minutes until soft and creamy.

Toast brioche buns. Spread spicy aioli, then layer sausage, scrambled eggs, caramelized onions, and avocado. Top with the other half and serve warm.