A photo like it (steak frites) · Sarah Stierch (CC BY 4.0) via Wikimedia Commons · source
The Paris beef house plate: an entrecôte with a crackling crust, a quick cognac and green-peppercorn sauce made in the same pan, and thin double-fried frites.
2 entrecôte (rib eye) steaks, about 1 inch (2.5 cm) thick, 10 oz (300 g) each
Salt
1 tbsp neutral oil, 1 tbsp butter
For the sauce: 1 tbsp butter, 1 shallot, finely chopped
1 tbsp green peppercorns in brine, drained and lightly crushed, plus 1 tsp black peppercorns, coarsely cracked
3 tbsp cognac
1/2 cup (120 ml) beef or veal stock
1/2 cup (120 ml) crème fraîche or heavy cream
For the frites: 1 1/2 lb (700 g) russet or Bintje potatoes
About 2 quarts (2 litres) peanut oil or beef fat, for frying
How to make it
Cut the potatoes into 1/4-inch (6 mm) sticks. Soak in cold water 30 minutes, drain and dry them very well on towels.
First fry: heat the oil to 300°F (150°C) and fry the potatoes in batches 4 to 5 minutes until soft and pale. Drain on a rack.
Salt the steaks and leave them at room temperature 20 minutes. Pat dry.
Heat a heavy pan over high heat, add the oil and then the steaks. Sear 2 1/2 to 3 minutes until a dark crust forms, turn, add the butter and baste, and cook 2 to 3 minutes more for saignant (rare to medium-rare), 125°F (52°C) inside. Rest them on a warm plate.
Sauce: pour off most of the fat, lower the heat, add the butter and shallot and cook 1 minute. Add both peppercorns, then the cognac (it may flame) and let it bubble away. Add the stock and boil 2 minutes, then the crème fraîche and any juices from the resting steaks; simmer until it coats a spoon. Season with salt.
Second fry: heat the oil to 375°F (190°C) and fry the frites 2 to 3 minutes until golden and crisp. Drain and salt at once.
Serve each steak whole with the sauce poured over half of it and a pile of frites.
Tips
Thin frites need the two fries; skip the soak and they stay limp.
Au Boeuf Couronné's puffed potatoes (pommes soufflées) are the restaurant trick; frites are the home one.