Toast walnuts; cool. Grind in blender or food processor until coarsely ground. Press spinach to drain excess water. Combine all ingredients except buns and topping, in a large bowl until moistened. Scoop 1/2 cup of burger mix and form into 1/2 inch thick patties. Place on parchemnt-lined baking sheet. Bake at 350 F in preheated oven until burger is lighlty golden around edges (20-25 minutes). Place burger on bun with toppings and condiments of choice.