In a big pan, melt the 1 Tbsp butter and sauté the spinach until it's soft. It will be a giant heap of greens at first, but it cooks way down and will fit in the pan when you're done!
Let the spinach cool and then squeeze out as much water as you can.
In a bowl, mix together the cooked spinach with the salt, pepper and nutmeg, and stir in the feta until it's combined. Set aside.
Preheat the oven to 375
Melt the stick of butter and set it aside. You'll need it handy for assembling the triangles.
Unroll the phyllo dough and cover it with a slightly damp cloth to keep it from getting brittle. Take what you need and keep the rest of the stack covered.
Carefully lay a phyllo dough square on your workspace, long side horizontal. Brush it with melted butter. Lay another sheet on top of it and brush that with butter.
With a sharp knife or pizza cutter, cut the dough into three strips.
Put a scoop of spinach mixture at the bottom of each strip. Then fold that section of dough up diagonally, enclosing the spinach, so it forms a triangle. Continue folding up to make triangles, like you'd fold a flag, until you reach the top of the dough. If you're having trouble figuring out how to fold it, here is a helpful video: https://www.youtube.com/watch?v=gVwA3i_tmKc&t=2s
If there's a bit of leftover dough on the triangle, fold it under. Lay the finished triangle on a baking sheet, seam side down. Brush with butter again.
Continue until the phyllo dough is gone. I made 18 pockets, two sheets thick, with one roll of phyllo dough, but you can change the proportions and make lots of smaller triangles if you like.
Bake about 25 minutes until golden brown. Let them sit in the pan for a moment before removing. Serve hot or cold.