Preheat the oven: Set to 375°F (190°C). Grease a 9x13-inch baking dish.
Blend the creamy base: In a large bowl, stir cream cheese, mayonnaise, and sour cream until smooth. Add lemon juice and Dijon. Mix until no lumps remain.
Season it right: Add Old Bay, garlic powder, onion powder, crushed red pepper, salt, and black pepper. Stir to combine.
Add the mix-ins: Fold in onion, celery, and green onions. Stir in 1 cup mozzarella and the cheddar.
Fold in the crab: Add the chopped imitation crab and gently mix to coat without breaking it up too much.
Assemble: Spread the mixture evenly in the baking dish. Sprinkle the remaining 1/2 cup mozzarella over the top.
Make the crunchy topping: In a small bowl, combine panko, melted butter, paprika, and Parmesan. Sprinkle evenly over the casserole.
Bake: Place on the center rack and bake 22–28 minutes, until the edges bubble and the top turns golden.
Rest and garnish: Let it sit 5–10 minutes to set. Top with extra green onions and a light squeeze of lemon if you like.
Serve: Scoop generous portions and enjoy with a crisp salad, steamed veggies, rice, or toasted bread.
