Add the pork tenderloin pieces to the slow cooker.
Layer the sliced onion and bell peppers on top of the pork.
In a small bowl, stir together the coconut milk, red curry paste, garlic, ginger, and salt until smooth.
Pour the coconut mixture evenly over the pork and vegetables.
Cover and cook on low for 6 hours.
Stir in the baby spinach during the last 10 minutes of cooking so it wilts without losing color.