Preheat the oven to 350°F (175°C). Place 1 1/4 lbs extra-large shrimp (peeled and deveined) in a single layer in a baking dish.
In a small saucepan over low heat, melt 1/4 cup butter. Add 1 Tbsp minced garlic (about 3 cloves) and sauté about 30 seconds until fragrant. Alternatively, melt the 1/4 cup butter in a microwave-safe dish and then stir in the minced garlic.
Remove the butter from heat and stir in 2 Tbsp fresh lemon juice.
Pour the garlic-lemon butter evenly over the shrimp in the baking dish.
Season the shrimp with salt and pepper to taste and 1/8 tsp red pepper flakes, if using. Gently toss the shrimp to coat with the sauce, then spread them into an even single layer.
Bake in the preheated oven until the shrimp are pink, opaque, and cooked through, about 9–13 minutes (shrimp should curl into a loose C shape and not be tightly curled or rubbery).
Remove from the oven, sprinkle with 2 Tbsp minced fresh parsley, and serve warm, spooning the sauce over the shrimp.