Add butter to cast-iron pan over medium-high heat and add onions once melted. Cook for about 10 minutes, stirring occasionally, until cooked down and caramelized.

Add thyme, beef consommé, and seasoning; cook down until liquid has almost evaporated, then set aside.

Lightly oil a cast-iron, stainless steel, or carbon steel pan or a griddle surface with oil and heat as close to 500 degrees Fahrenheit as possible.

Place frozen burgers directly on cooking surface; cook for 3 minutes, then flip and cook for one additional minute. While burgers cook, toast buns.

Season with salt, top with caramelized onions, and continue cooking for an additional minute.