Spread 1 tablespoon of butter on one side of both pieces of bread.

Peel and slice the onion thin. Melt the remaining butter in a frying pan over medium heat and add the onions. Fry for about 5 minutes until caramelized, then add 2 tablespoons of water and cook until tender.

In a bowl, mash ½ avocado with the salt and black pepper until smooth. In another bowl, mix the mayonnaise and sriracha together.

Flip the buttered bread so the buttered side is facing down. Spread the mashed avocado on one slice and the spicy mayo on the other.

For scrambled eggs, beat eggs with salt and pepper, then cook in the pan until firm. For fried eggs, crack eggs directly into the pan and fry until desired doneness.

In the pan, place the bread slice with avocado facing down, stack cheese on top, add cooked eggs, layer sautéed onions, then place the mayo-covered slice on top.

Fry the sandwich until the bottom is golden brown and crispy, then carefully flip to fry the other side until toasted and the cheese melts.

Slice the sandwich in half and serve hot.