Preheat and prep: Set oven to 375°F (190°C).
Lightly grease a 1 1/2–2 quart baking dish.
Cook the shrimp (if using raw): Warm olive oil or butter in a skillet over medium heat. Add shrimp, a pinch of salt, and pepper. Cook 2–3 minutes until just pink.
Cool slightly, then chop into small, bite-size pieces.
Soften the base: In a large bowl, beat cream cheese until smooth. Mix in sour cream and mayonnaise until creamy.
Add seasonings: Stir in garlic, lemon juice, Old Bay, Worcestershire, hot sauce (if using), and half of the green onions. Taste and adjust salt and pepper.
The mixture should be well seasoned now.
Stir in cheeses: Fold in cheddar and Parmesan, reserving a small handful of cheddar for the top.
Fold in seafood: Gently add the chopped shrimp and lump crab. Try not to break up the crab too much. Fold just until combined.
Transfer and top: Spread the mixture into the baking dish.
Sprinkle the reserved cheddar over the top for a golden crust.
Bake: Bake 18–22 minutes, until the edges are bubbling and the top is lightly browned. Avoid overbaking to keep the seafood tender.
Finish and garnish: Let the dip rest for 5 minutes. Sprinkle with chopped parsley and the remaining green onions.
Serve: Enjoy warm with crackers, toasted baguette slices, or veggie sticks.
A squeeze of fresh lemon at the table is a nice touch.