Generously spray the sides of a 7-quart slow cooker with cooking spray.
Add in the broth, quinoa, onion, lemon juice, lemon zest, salt and pepper and stir until well combined.
Cut the top of the head of garlic off, exposing the cloves but keeping the head in tact. Place 2 small pieces of tinfoil on top of each other and place the garlic in the center, cut-side up. Rub 1/2 tsp of the butter all over the top of the exposed cloves. Then, wrap the clove up in the tinfoil to make a sealed package. Place it into the center of the quinoa mixture.
Cover and cook on high until the liquid is absorbed and the quinoa is creamy, about 2-3 hours. Mine was perfect at about 2 and a half hours.
Once cooked, remove the garlic and set aside to cool. Stir the remaining butter into the quinoa. Then, place the spinach on top of the quinoa in a pile, not stirring it in. Finally, place the raw shrimp on top of the spinach.
Cook until the spinach is wilted and the shrimp are cooked and opaque, about another 15 minutes.
Press the garlic out of the cooled and stir into the quinoa. *
*I LOVE garlic, so I used the whole head! But, you may not want to. So, add half, taste and add more if you want!