Melt 2 tablespoons butter over medium heat, then sauté onions and garlic until translucent.
Add spinach and cook until wilted. Remove onion and spinach from pan. Set aside.
Melt remaining 2 tablespoons of butter, then cook shrimp for 5 minutes, flipping half way through. Shrimp will be nice and pink when cooked to an internal temperature of 120°F.
Add the spinach and onion back to the pan and stir together with the shrimp. Remove from heat. Serve over grains.