Preheat oven to 200 degrees C. Line a rimmed 22cm x 36cm baking dish with baking paper and grease with cooking spray.
Place asparagus in the prepared baking dish and drizzle with 1 tablespoon olive oil. Sprinkle with garlic, 1 teaspoon salt and 1/2 teaspoon pepper; toss until well coated. Arrange asparagus in a single layer.
Bake in the preheated oven until slightly tender, about 6 minutes.
Remove from oven and place prawns on one side of the pan. Drizzle with remaining 1 tablespoon olive oil, 1 teaspoon salt, 1/2 teaspoon pepper and smoked paprika; toss until well coated. Place prawns in a single layer next to the asparagus. Top asparagus and prawns with cubed butter.
Roast in the preheated oven until prawns opaque, about 6 minutes.
Remove pan from oven and drizzle with lemon juice.