Adjust two oven racks, one in the middle and the other in the upper third of the oven, and preheat oven to 400°F.
Lightly brush or spray two baking sheets with melted butter.
Remove the phyllo roll from its packaging and place it on a cutting board (do not unroll yet). Using a sharp knife, cut the roll into 3 equal sections, each about 3 inches wide.
Rewrap two sections in the original plastic or cover with a lightly damp towel to prevent drying.
Unroll the remaining section on a cutting board. Remove two sheets and cover the rest with a damp towel.
Place the two sheets on top of each other and lightly brush the top sheet with melted butter.
Place about 1½ tablespoons of the spinach mixture near the bottom corner of the strip closest to you, shaping it into a diagonal line..
Fold the sheets over the filling in a triangle shape and continue folding up the strip until you reach the end, forming a neat triangle.
Place the triangle on the prepared baking sheet and brush the top with butter.
Repeat with the remaining phyllo and filling. You should end up with about 24 triangles.
Bake on the middle rack for 10 minutes, or until the bottoms are golden. Then move the tray to the upper rack and bake for another 5–8 minutes, until golden brown and crisp.
Remove from the oven and let cool slightly before serving.