Step 1: Preheat your oven to a cozy 375°F.
Step 2: In a medium to large mixing bowl, combine your chopped spinach (remember to squeeze out all that excess water if it was frozen!), the softened cream cheese, sour cream, mayonnaise, chopped green onions, Old Bay seasoning, lemon pepper seasoning, mozzarella cheese, and half a cup of your Muenster cheese. Don't forget a little sprinkle of pepper! Mix everything thoroughly until it’s all wonderfully combined and super creamy. Taste and adjust any seasonings if needed. Set this mixture aside.
Step 3: In a small pan, heat your cooking oil over medium heat. Add your diced onion and garlic paste (or minced garlic). Sauté these until they become translucent and beautifully soft – this usually only takes a few minutes.
Step 4: Add your medium-sized, peeled, and deveined shrimp to the pan with the onions and garlic. Cook them for just about one minute, or until they just begin to turn pink. Do not overcook them here, as they'll continue cooking in the oven later.
Step 5: As soon as those shrimp are just blushing pink, remove the pan from the heat to stop the cooking process.
Step 6: Carefully combine the cooked shrimp, onion, and garlic mixture with your prepared creamed spinach mixture. Give it a gentle stir to make sure the seafood is evenly distributed throughout the creamy base.
Step 7: Transfer your glorious seafood dip into an oven-safe dish (a shallow baking dish or even a cast iron skillet works wonderfully here). Then, sprinkle the remaining ½ cup of Muenster cheese all over the top.
Step 8: Pop your dish into the preheated oven and bake for 20 minutes. It's ready when the dip is heated through and the shrimp are perfectly cooked, and the edges might even be starting to bubble!
Step 9: For that ultimate golden, bubbly top, change your oven setting to broil. Broil the dip for just 30 seconds to 1 minute, keeping a very close eye on it, until the cheese is beautifully golden brown and actively bubbling. Once it's perfect, carefully remove it from the oven, let it cool for just a minute or two, and then serve immediately.