Mince garlic, slice onion, and chop pineapple.
Place chicken into a slow cooker. Season with salt and pepper, drizzle with olive oil, and spritz with lemon juice. Add garlic and onion. Cook on low for 6 hours.
Remove chicken from slow cooker and shred with two forks. Toss back into slow cooker and add pineapple and barbecue sauce; stir to combine and then place cover back on. Preheat oven to 425°F.
Spray two tortillas with cooking spray. Place coated sides down onto baking sheet. Top each with some of the chicken mixture, spinach, and cheese; cover with remaining tortillas, spraying the tops with cooking spray.
Bake for 10 minutes or until golden brown. Cut into wedges and serve.