Pound garlic, shallot, black peppercorns, lavender, olive oil, and pomegranate molasses in a mortar and pestle to form a rough paste. Rub paste onto lamb chops and allow to rest, covered, at room temperature for 1 1/2 hours or in the refrigerator for up to 24 hours. Allow meat to return to room temperature before cooking.
Combine onions, salt, pepper, and olive oil in a heavy saute pan and cook, stirring frequently, over medium heat until the onions begin to soften. Add vinegars. Cover surface of onions with a disk of parchment paper and cover pan with a tight fitting lid. Reduce heat to very low and allow to cook undisturbed for 1 hour. Check seasoning and keep warm.
Combine celery root and milk in a saucepan. Bring to a simmer, partially covered, and simmer until tender, approximately 30 minutes. Using a slotted spoon, remove celery root from pan and put through the fine disk of a food mill. Beat in the salt, pepper, and olive oil. Use leftover cooking liquid as necessary to adjust consistency. Keep warm.
Final assembly: Scrape excess marinade from the lamb chops and season with sea salt. Roast chops in a wood fired oven or grill over a hot charcoal fire to your desired degree of doneness. Mound 1/4 of the puree in the center of each of 4 warm plates. Lean 2 chops against each mound and put a portion of onion marmalade on the plate near them. Garnish with a few pomegranate seeds.