Season shrimp to taste with salt. In a medium skillet, heat olive oil over medium-high heat. Add half the shrimp and sear for about 1 minute per side until golden and opaque. Transfer to a plate. Repeat with remaining shrimp.
Add butter to skillet and melt. Add garlic and sauté for 1 minute. Add black pepper and toast for about 1 minute, until fragrant. Stir in cream and spinach and cook until sauce thickens slightly. Add shrimp and cook for 2 minutes.
Adjust seasoning with salt, then garnish with chives and crushed red pepper flakes and serve.