Preheat your oven to 325 degrees F. Spread the walnut halves on a baking sheet in a single layer and toast for 7 to 8 minutes until fragrant and lightly golden. Let them cool completely.
In a microwave-safe bowl, combine the chopped sugar-free dark chocolate and coconut oil. Microwave in 30-second intervals, stirring after each one, until the chocolate is fully melted and glossy.
Stir the cinnamon into the melted chocolate until combined. Add the toasted walnut halves and fold gently until every walnut is coated in chocolate.
Line a baking sheet with parchment paper. Drop spoonfuls of the mixture onto the parchment, forming small clusters of 3 to 4 walnuts each. Sprinkle each cluster with a pinch of flaky sea salt.
Refrigerate the clusters for at least 30 minutes until the chocolate is completely set and firm. Store in an airtight container in the refrigerator for up to 2 weeks.