Place the cedar planks in water and soak for at least 1 hour before grilling so they don’t burn too quickly.
Place 4 6-ounce salmon fillets in a 9x13 baking dish or shallow dish. Drizzle with 1/3 cup olive oil, then top evenly with 2 cloves garlic , 2 sprigs fresh rosemary, 1 tablespoon lemon zest, and 3 tablespoons lemon juice.
Refrigerate the salmon for up to 20 minutes. In the meantime, preheat the grill to medium-high heat and set it up for indirect grilling. Place the soaked cedar planks over the cooler side of the grill.
Remove the salmon from the fridge and place 2 fillets on each soaked cedar plank, leaving a little space between them so they cook evenly.
Grill with the lid closed for 15-20 minutes, depending on the thickness of the fillets, until the salmon flakes easily and reaches your preferred doneness. For fully cooked salmon, aim for 145°F. *Keep a spray bottle of water nearby in case of flare-ups.
Remove the salmon from the grill and let it rest for 5 minutes. The fish will continue to cook slightly on the hot plank as the juices settle. Carefully use a spatula to lift the salmon off the planks and serve warm.
Notes
Look for grilling cedar planks near the seafood counter, in the grilling aisle, at kitchen stores, or online. Just make sure they’re untreated and labeled for cooking.
One lemon should be enough for the zest and juice, but if it's on the small side, you may need two.