Bake burgers according to package directions.

While the burger is baking, heat the olive oil up in a skillet and sauté onions on low heat until caramelized and golden.

Next, make the cheese sauce. Melt the butter in a pot and stir in the flour. Mix well until large clumps form. Pour in the milk and cook over medium heat until it begins to thicken. Whisk in the cheese. Keep warm and whisk before serving.

Assemble the burgers with spring mix, the patty, cheese sauce, and top it off with the caramelized onions.