Add walnuts and cook for 2-3 minutes, stirring often.
Add sea salt and maple syrup and continue to cook, stirring constantly, until walnuts are caramelized (3-4 minutes).
Use a slotted spoon to transfer the nuts to a sheet of parchment paper. Use back of spoon to spread nuts apart, then allow to cool completely (5-10 min) before serving.
Store in an airtight container for up to a few days. If they lose their crisp after 2-3 days, just pop them into a 375 degree oven on a parchment lined sheet for a few minutes.
Notes
When caramelizing the nuts, watch them very closely - the line between caramelized and burning is a very fine one!