Melt the butter with thyme, garlic and Espelette pepper. Place the hay in a pot and light it. Put out the fire with the melted butter and cover the pot with a lid. Strain the butter just before serving.
Portion the sirloin as shown by your chef and fry the pieces golden brown in a hot pan. Baste with butter and cook in an oven at 100°C until a core temperature of 56°C. Let the meat rest. Season with salt and pepper and slice the meat. Brush the it with melted butter and grate the cheese.
Soak the peas in cold water overnight. Boil the peas with mustard seed, thyme and garlic until the peas are almost soft. Drain the peas and remove the thyme and garlic. Toss the peas in the grilled butter, season with salt and pepper.
Peel the onions and cut them into thick slices. Place the slices along with the remaining ingredients in a vacuum bag. Seal the bag and cook the onions at 85°C for 15 minutes or until soft. If you don’t have access to a vacuum machine you can let the onions simmer on the stovetop instead. Keep the liquid for the jus. Place the onion rings on a tray and blast them with a blowtorch.
Chop the onion and apple and fry until golden brown. Add the stock and reduce by half. Roast the hay 10 minutes in the oven at 170°C. Add the hay to the reduced stock and season with salt and the liquid from the baked onion. Strain the jus.
Peel the celeriac and slice 1/3 of it thinly using a mandolin. Cut the slices into thin matchsticks. Deep fry the sticks until crispy. Cut the remaining celeriac into cubes and cook it in vegetable oil until soft. Mix to a smooth cream in a blender with a little of the oil. Season with salt and vinegar.
Bring the water to a boil with sugar, salt and vinegar. Cut the endives into 4 and pickle them until serving.