Add butter to your saucepan on medium heat. Once sizzling, add onions and stir until well coated. Season with a pinch of kosher salt.
Lower the heat to low and stir every few minutes for 20 minutes. Cook for another 10 minutes, stirring more often until dark brown.
Cook the fries based on the bag instructions.
When fries are cooked, add sliced cheese and put back in the oven until melted. Top with caramelized onions and scallions.
Top with or dip into a spicy mayonnaise (1/2 cup mayonnaise + 2 tablespoons Sriracha sauce or more to taste)