Generously spray the sides and bottom of a 7-quart slow cooker with cooking spray.
Add 2 cups reduced-sodium vegetable broth, 1 cup uncooked quinoa, 1/2 cup chopped onion, 1/4 cup freshly squeezed lemon juice, zest of half a lemon, 1 teaspoon salt, and black pepper to taste. Stir until evenly combined.
Prepare the garlic: cut the top off 1 head of garlic to expose the cloves, place the head cut-side up on two small squares of aluminum foil, rub 1/2 teaspoon butter over the exposed cloves, then wrap the garlic tightly in the foil to make a sealed packet.
Nestle the foil-wrapped garlic packet into the center of the quinoa mixture in the slow cooker.
Cover and cook on HIGH for 2 to 3 hours, until the liquid is absorbed and the quinoa is creamy (about 2 to 2 1/2 hours is typical).
Carefully remove the foil-wrapped garlic from the slow cooker and set it aside to cool until it can be handled safely.
Stir the remaining 2 tablespoons butter into the cooked quinoa until melted and evenly distributed.
Pile 6 cups packed spinach on top of the quinoa without stirring it in, then place 1/2 pound raw shrimp (tail-on, peeled and deveined) on top of the spinach in a single layer if possible.
Cover and cook on HIGH until the spinach is wilted and the shrimp are opaque and cooked through, about 15 minutes.
Unwrap the garlic, press the roasted cloves out of their skins, add as much of the roasted garlic as you like to the slow cooker, and stir it through the quinoa and shrimp mixture.
Serve warm.