Melt 55g duck fat over low heat and mix with thyme, rosemary, and oregano.
Pour the herb fat over the steaks and marinate for 10 minutes.
Heat a barbecue or griddle pan over medium-high heat.
Cook steaks to preference, season with salt and pepper, and rest in a warm place.
Combine softened butter, mustard, garlic, parsley, and capers then chill until set.
Boil whole potatoes in salted water until just cooked, then drain and cool.
Melt remaining 110g duck fat in a saucepan over low heat.
Slice potatoes into wedges and coat thoroughly in the melted duck fat.
Grill wedges until crispy, brushing with extra duck fat as necessary.
Slice meat, top with melted herb butter, and serve with wedges and salad.