Prep the Oven and Parchment: Preheat the oven to 400°F (200°C). Tear a sheet of parchment paper roughly 12x16 inches. Fold it in half crosswise, crease it, and open it back flat on a rimmed baking sheet.
Assemble the Packet: Place the 3 lemon slices in a single layer slightly to one side of the center crease. Pat the keta salmon filet dry with a paper towel and lay it directly over the lemon slices. Season the top of the salmon with salt and pepper to taste. Place the sprig of fresh rosemary directly on top of the seasoned filet. Coat the salmon and rosemary with a light, even spray of olive oil.
Fold and Crimp: Fold the empty half of the parchment over the salmon so the outer edges line up. Starting at one corner, make small, tight, overlapping diagonal folds along the rounded open edge. Work your way completely around the packet, pressing firmly with each pleat to ensure an airtight seal, and tuck the final fold under the packet.
Bake and Serve: Bake at 400°F for 11 to 13 minutes. Because keta salmon is naturally lean, pull it right as the paper fully puffs and the thickest part reaches 125°F–130°F to keep it tender. Transfer the packet directly to a serving plate and cut a slit across the top with a knife or kitchen shears, watching out for hot escaping steam.