Preheat your oven to 400 degrees F. Line a half-sheet pan with foil or parchment paper for easy cleanup.
Rinse the salmon and feel for any bones to remove. Pat the fish dry with paper towels. If there are any parts of the salmon that are very thin compared to the rest, use kitchen scissors or a knife to trim them off so you can bake the thin pieces for less time than the rest.
Place the salmon on your lined pan. Spray the salmon with cooking spray and sprinkle on black pepper.
Scoop a tablespoon of pesto onto a large piece of salmon, a couple inches from one end. Scoop another tablespoon of pesto onto the salmon, near the opposite end. (For best results, do not spread the pesto until after the fish is baked. See notes.) Continue placing two tablespoons of pesto onto each large piece of salmon. For any small thin, pieces, you'll probably just need one tablespoon of pesto.
Bake your thick pieces of salmon for 20 minutes, or until it can be flaked with a fork. If you have any thin pieces, bake them for five minutes less.
After the salmon is baked, use a knife or the back of a spoon to spread the pesto across each piece. If your salmon has skin on the bottom, use a metal spatula to gently lift the cooked salmon off the skin before serving your fish. Garnish each piece of salmon with a slice of lemon, if desired.
Store leftovers in the refrigerator for up to three days or freeze in a single layer in a freezer bag. Reheat leftovers in the microwave.
The prep time doesn't include making the pesto. If you're making Mom's Pesto with Pecorino Romano, you'll need another 25 minutes. You also could make this with Spinach-Walnut Pesto.Bake your salmon topped with dollops of pesto and then spread the pesto across the fish afterward, to ensure your pesto has a more vibrant color.