Saute onion and garlic in olive oil over lowest heat, partly covered, for 20 minutes. Do not allow to color.
Blanch watercress and herbs in boiling water for 1 minute. Drain,k refresh under cold water, drain againand pat dry in paper towels.
Place all ingredients except butter and egg yolks in food processorl. Process to a smooth paste. Add butter and process until well combined. Add egg yolks and salt and pepper to taste. Process just until eggs have been well blended. Mixture should be glossy.
Meal: Soak 12 wood skewers in water for one hour. Distribute scallops evenly on 6 of the skewers. Brush with olive oil and marinate in refrigerator 1 hour.
Peel and slice onions 1 inch thick. Divide slices and skewer.
Brush skewered onions with olive oil. Salt and pepper. Grill over hot coals in spot where heat is low for 5 minutes. Turn over, salt and pepper, and grill for 3 minutes.
Grill scallops over hot coals for 2 minutes. Turn, salt and peper to taste, and grill another minute, until opaqe all the way through.
Remove scalloops and onions from the heat, top with Repulse Bay Hotel Butter and serve.