Heat oil over medium heat in a large skillet and add onions, stirring until translucent, about 7 minutes. Add shrimp, scallops, crab meat and tuna. Cook until shrimp are pink and the scallops are opaque, 8 to 10 minutes. Season with salt, pepper, and Old Bay Seasoning. Remove from heat.
Whisk together the mayonnaise, mustard, garlic powder, oregano, turmeric and sugar in a large bowl. Mix in the bell pepper, 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 flavors blend.