Lay out two large sheets of aluminum foil, allowing enough to enclose the salmon completely. Arrange the 3 sprigs fresh rosemary in the center.
In a small bowl, stir together the 40 g unsalted butter, melted, ½ teaspoon salt, ¼ teaspoon black pepper, 4 garlic cloves, minced, and 1 tablespoon lemon juice.
Place the 500 g salmon fillet skin-side down on the rosemary. Spoon the garlic butter over the flesh, spreading the garlic across the surface, then arrange the 3 lemon slices on top.
Fold the foil around the salmon and seal the edges to form a closed packet. Place it on a baking sheet.
Bake for 10 minutes, then carefully open the packet and check the thickest part. The salmon should flake easily and reach an internal temperature of 63 °C. If needed, close the foil and continue baking, checking again until cooked through.
Open the foil away from your face to release the steam. Divide the salmon into four servings and spoon the cooking juices over the fish. Serve immediately.
Notes
Cooking time: The tested baking time is 10 minutes. Fillet thickness can affect cooking time, so check the thickest part before serving. The total time excludes oven preheating. Storage: Refrigerate leftovers in an airtight container for up to 2 days, keeping a little of the cooking liquid with the salmon. Reheating: Remove all foil before microwaving. Reheat individual portions on medium-low power until hot throughout, or use a covered skillet with a splash of water and heat gently until hot throughout.