Heat oil over medium heat in a large frypan and add onions, stirring until translucent, about 7 minutes. Add prawns, scallops, crab meat and tuna. Cook until prawns are pink and the scallops are opaque, 8 to 10 minutes. Season with salt, pepper and bay seasoning. Remove from heat.
Whisk together the mayonnaise, mustard, garlic powder, oregano, turmeric and sugar in a large bowl. Mix in the capsicum, celery and chopped eggs.
Add seafood and toss until evenly combined. Garnish with eggs slices and sprinkle with paprika.
Cover and refrigerate for 2 to 3 hours or overnight before serving to let flavours blend.
How to boil eggs
Place eggs in a saucepan and cover with cold water. Bring water to a boil and immediately remove from heat. Cover and let eggs stand in hot water for 10 to 12 minutes. Remove from hot water, cool, peel then dice.