Pat duck breasts dry and place in a shallow dish.
Peel lemons for zest ringlets and squeeze the juice.
Marinate duck with zest, juice, and 250ml limoncello for 30 minutes.
Remove duck from marinade, dry with paper, and season with salt and pepper.
Melt and clarify 50g butter in a pan, then add olive oil.
Seal duck breasts skin-side down in the hot pan until browned.
Reduce heat and cook for 12 minutes, turning occasionally.
Increase heat, add marinade, and reduce until the sauce thickens.
Rest duck for five minutes, slice, and drizzle with the sauce.
Boil carrots and celeriac, then strain through muslin.
Puree vegetables with olive oil and seasoning using a hand blender.
Serve the sliced duck with the vegetable mash.