Preheat grill to MEDIUM-HIGH.
In a large bowl, combine ground turkey, salt, pepper and onion powder. Form mixture into 4 patties then place them on the preheated grill; cook for 5 to 10 minutes per side or until burgers are no longer pink in the center.
Place mushroom caps in a large shallow dish; coat both sides with half of the coconut oil then season with salt and pepper. Brush grill grate with remaining coconut oil then place mushroom caps on grill; cook for 3 to 4 minutes per side or until softened.
On 4 of the mushroom caps, layer a burger, sliced tomato, avocado and onion, then chopped spinach.
Top each with remaining mushroom caps and serve.