In a large frying pan, heat oil over medium heat. Add onions and cook until tender but not browned. Stir in garlic and chopped spinach, salt and pepper. Cook until spinach just starts to wilt. About 3 minutes.
Remove from heat. Add cooked rice and stir in lemon zest until well combined.
Spread mixture evenly over salmon fillets and sprinkle with cheese and walnuts.
Roll up gently and secure with toothpicks or twine.
Place salmon on a parchment lined baking sheet and bake at 375 F for about 15-20 minutes, or until fish is cooked through.
Transfer to a cutting board and cool for 10 minutes before slicing into pinwheels.