Preheat grill to medium. Brush salmon with olive oil and season with salt and pepper to taste.
Remove rosemary needles from sprig. Thinly slice one lemon. Cut the other two lemons in half.
Sprinkle rosemary on fish, drizzle on a little maple syrup and place cut lemon slices on top.
Place salmon skin side down on foil. Fold foil up on sides to create walls keeping juices in. Close lid and grill salmon for 8 to 10 minutes or the rule of thumb is 10 minutes per 1 inch of thickness. It's normally less however since salmon tends to be uneven.
Put cut lemon directly on hot grill and cook 3-4 minutes or until lightly charred.
Serve with grilled lemon squeezed on top.