Mix the liqueur, honey, prosecco, lemon peel, mint, chilli, and tarragon in a large bowl.
Marinate the cod fillets in the refrigerator overnight.
Remove the cod from the marinade and season with salt and pepper.
Simmer red onions, sugar, beetroot, and vinegar for 45 minutes until jam-like.
Pass the onion mixture through a sieve to remove lumps and let it cool.
Grill the cod over low-medium heat for 15 to 20 minutes until glazed and firm.
Char the lettuce quarters using a blow torch and dress with olive oil and lemon juice.
Serve the cod on a bed of onion jam with the lettuce and pea shoots.