Grate potatoes in a food processor fitted with a grater attachment. Transfer to a bowl. Trim the brussels sprouts, reserving the outer leaves. Thinly slice the brussels sprouts in a food processor fitted with the slicing attachment.
Heat butter and 1 tablespoon oil in a large frying pan over medium-high heat. Add the leek. Cook, stirring, for 3 minutes or until softened. Add potato. Cook, stirring occasionally, for 10 minutes or until golden. Add the brussels sprouts, garlic and remaining oil. Season well. Cook, stirring occasionally, for 10-12 minutes or until golden and caramelised. Stir in reserved brussels sprout leaves and 3 tsp lemon juice.
Meanwhile, combine the crème fraîche, mustard and remaining lemon juice in a bowl. Season.
Spray steaks with oil. Preheat a chargrill pan on medium-high. Season. Cook steak, turning, for 4-5 minutes for medium or until cooked to your liking. Set aside for 2 minutes to rest. Serve steaks with hash and mustard crème fraîche.