Mix the duxelles and ground beef well, then cook a small amount to taste. Depending on how strongly you season your duxelles, you may or may not want to season the burgers. Generally I season burgers ahead of time and mix for a better texture, flavor, and shelf life raw. If you can, leave the burgers uncovered in the fridge for at least a few hours before cooking for the best crust.
Season the burgers lightly with salt and pepper, then cook over a hot fire or in a dangerously hot cast iron skillet to your desired temperature. Medium is plenty.