Using long-handled tongs, moisten a paper towel with cooking oil and lightly coat the grill rack. Grill salmon skin side down, covered, over medium heat or broil 4 in. from the heat for 12-14 minutes or until the salmon flakes easily with a fork.
Meanwhile, in a large skillet, saute spinach in 1 tablespoon butter until tender; remove from the pan and keep warm. In the same skillet, saute garlic in remaining butter for 1 minute. Stir in the lemon juice, salt and pepper.
Place spinach on serving plates; top with salmon. Drizzle with butter mixture and sprinkle with hazelnuts.