Start by preparing your fillets. Take out of the refrigerator and season with fine sea salt and freshly ground pepper on both sides. Allow to sit at room temperature while you heat the grill to medium-high heat.
Heat 1 tablespoon unsalted butter and 2 tablespoons extra virgin olive oil in large skillet. Add leeks, mushrooms and garlic. Saute until soft, approximately 5-10 minutes.
When the grill is heated, grill for 4-5 minutes on each side. Remove from the grill and allow to set for a minimum of 3 minutes.
To serve, spoon leeks and mushrooms on a plate, top with grilled beef fillet and top with a spoonful of brandy garlic butter.
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Notes
Leeks get a lot of dirt and grime stuck between the layers. After slicing, place in a bowl of water and gently push the rings apart. Rinse well to remove all muck.