Preheat oven to 375°F.
Defrost spinach in microwave ( about 5 minutes on high )
Place in strainer and remove as much water as possible, coarsely chop.

Melt one tablespoon butter in a large heavy skillet over moderate heat, add onions and saute for a minute, then add spinach, dill and flour, cook an additional 4 minutes.
Remove from heat and cool a few minutes.
Using fork, whip eggs and add with crumbled ( or chopped ) feta, nutmeg and lemon Season with salt and pepper to taste. Let filling cool.
Melt 1/2 stick of butter in a small saucepan, then cool.
Cover phyllo stack with 2 overlapping sheets of plastic wrap and then a dampened kitchen towel.
Take one phyllo sheet from stack and arrange on a work surface with a long side nearest you (keeping remaining sheets covered) and brush with some butter. Top with another phyllo sheet and brush with more butter. Cut buttered phyllo stack lengthwise into 4 strips.

Put a heaping teaspoon of filling near one corner of a strip on end nearest you, then fold corner of phyllo over to enclose filling and form a triangle. Continue folding strip (like a flag), maintaining triangle shape. Put triangle, seam side down, on a large baking sheet and brush top with butter. Make more triangles in same manner, using all of phyllo.

Bake triangles in middle of oven until golden brown, 20 to 25 minutes, then transfer to a rack to cool slightly.