Step #1 Melt 1 tbsp butter in a 12-inch heavy skillet over moderate heat, stirring, until wilted & tender, then cook spinach, about 4 mins.
Step #2 Remove from heat & cool, about 10 mins.
Step #3 Squeeze handfuls of spinach to remove as much liquid as possible, then coarsely chop.
Step #4 Transfer to a bowl & stir in the feta, 1/2 tsp salt, nutmeg, & 1/2 tsp pepper.
Step #5 Preheat oven to 375°F
Melt remaining 1 stick butter in a small saucepan, then cool.
Step #6 Cover up phyllo stack with 2 overlapping sheets of plastic wrap & then a dampened kitchen towel.
Step #7 Take 1 phyllo sheet from stack & arrange on a work surface with a long side nearest you (keeping remaining sheets covered) & brush with some butter.
Step #8 Top with another phyllo sheet & brush with more butter.
Step #9 Cut buttered phyllo stack crosswise into 6 (roughly 12- by 2 3/4-inch) strips.
Step #10 Put a heaping tsp of filling near 1 corner of a strip on end nearest you, then fold corner of phyllo over to enclose filling & form a triangle.