Preheat oven to 400° F. Prepare a 1.5-2 quart casserole dish by spraying with cooking spray.
In a large skillet, add butter over medium-high heat. Once melted, add onion and cook until translucent. Add shrimp and cook for 1 minute per side. (Barely pink. They'll cook more in oven.)
Reduce heat to low, remove shrimp and place on a cutting board. Roughly chop into smaller pieces.
Add garlic to pan and cook, stirring often, for 1 minute. Increase heat to medium and add half and half, cream cheese, gruyere, 1/2 cup parmesan, lemon pepper seasoning, lemon juice and spinach. Stir until cheese is melted and everything is combined well. Add shrimp back to pan and stir in.
Pour mixture into prepared dish and top with remaining 1/4 cup parmesan. Bake for 15-20 minutes or until bubbly and golden. (Broil for a couple minutes if it isn't as brown as you like on top.)
Serve hot with sliced, toasted baguette or pita chips.