1. Prep
   • Preheat your oven to 375 °F (≈190–191 °C).
   • Grease a 2½ -quart casserole dish (or deep 10 -inch skillet, or 9×9 square pan) with butter so nothing sticks.
2. Poach the shrimp
   • Heat a large pot of water until it reaches a vigorous, rolling boil.
   • Add 1 Tbsp salt + 1 Tbsp Old Bay Seasoning to the boiling water.
   • Add the shrimp and boil just 30 seconds (or until shrimp turn opaque and are just cooked through). Don’t overcook.
   • Immediately strain them out and transfer to a large mixing bowl. Let cool a few minutes.
   • Drain off any extra liquid (tilt the bowl, pour off, or pat bottom with paper towels).
3. Mix the shrimp filling
   • Into the shrimp bowl, add the mayonnaise, garlic powder, and black pepper. Stir to coat shrimp evenly.
   • Add the shredded mozzarella and cheddar; stir gently so the cheeses meld with the shrimp mixture.
4. Assemble in the baking dish
   • Transfer the shrimp​-cheese mixture into your prepared casserole dish. Spread it evenly.
5. Add the crumb topping
   • In a small bowl or measuring cup, melt 1 Tbsp butter.
   • Stir in the panko bread crumbs until they are evenly moistened.
   • Sprinkle the crumb mixture over the top of the shrimp layer.
6. Bake & broil
   • Place the casserole (uncovered) in the 375 °F oven. Bake for 13–15 minutes, or until the center reaches ~145 °F (63 °C) on an instant​-read thermometer.
   • Then switch oven to broil and broil the top for 1–2 minutes (watch carefully) to brown the crumbs lightly.
7. Rest & serve
   • Remove from oven and allow the casserole to rest for about 15 minutes (this helps it set).
   • Serve warm or at room temperature.

Note: If excess liquid is present in the bottom after baking, gently absorb it with a paper towel inserted between shrimp and dish base or tilt and pour off.