Remove the stems from the kale and break into pieces. Wash the leaves thoroughly under running water. Dry in a salad spinner, then dry with paper towels. Transfer to a very large bowl.
Add olive oil and gently massage the leaves to get the oil everywhere.
Lay the leaves in a single layer on a large baking sheet with some space between each leaf. Do not overcrowd otherwise they will steam instead of crisping up. You may need to do this in two batches.
Bake for 20 minutes, then rotate the pan and bake for another 15-20 minutes until crispy. Sprinkle with sea salt.