Preheat your oven to 400 degrees Fahrenheit (200 degrees Celsius). Line a large baking sheet with parchment paper for easy cleanup.
In a small bowl, whisk together the melted butter, minced garlic, salt, black pepper, and red pepper flakes, if using. This is your garlic butter sauce.
Place the broccoli florets on one side of the prepared baking sheet. Drizzle half of the garlic butter sauce over the broccoli and toss to coat evenly. Spread the broccoli into a single layer.
Place the shrimp on the other side of the baking sheet. Drizzle the remaining half of the garlic butter sauce over the shrimp and toss gently to coat. Spread the shrimp into a single layer, avoiding overcrowding.
Roast for 8 to 10 minutes. The shrimp should be pink and opaque, and the broccoli should be tender-crisp. Do not overcook the shrimp.
Remove the sheet pan from the oven. Drizzle the fresh lemon juice over the shrimp and broccoli. Sprinkle with fresh parsley.
Serve immediately directly from the pan or transfer to plates.
Notes
For best results, ensure your shrimp are completely thawed and patted dry before tossing them in the sauce. Excess moisture steams the shrimp instead of roasting it.
If you prefer a stronger lemon flavor, add 1 teaspoon of lemon zest to the garlic butter sauce before roasting.
If your shrimp and broccoli pieces are very different sizes, you can roast the broccoli for 5 minutes alone before adding the shrimp to the pan.