Heat oil in a medium sauté pan over medium-high heat. Sauté onions and garlic for 5 minutes or until translucent. Transfer to a large bowl and add spinach, feta, nutmeg, and cinnamon and stir to combine.

Take one phyllo sheet from stack and arrange on a work surface with the long side closest to you. Brush with melted butter. Top with another sheet and brush with more butter. Top with one more sheet and brush with butter. Cut stack crosswise into 8 strips.

Preheat oven to 375°F.

Put a heaping teaspoon of filling near one corner of a strip on the end closest to you. Fold corner of phyllo over to enclose filling and form a triangle. Continue folding the strip, like a flag, maintaining the triangular shape. Put triangle, seam side down, on a lightly greased large baking sheet and brush the top with butter. Repeat with remaining strips and filling. Repeat with remaining three phyllo sheets to make 16 triangles total.

Bake until golden brown, 20 to 25 minutes, and transfer to a wire rack to cool slightly.

TIP: Cutting Lengthwise vs. Crosswise Cutting an ingredient "lengthwise" means to slice it the long way, the length of the ingredient from "pole to pole." Cutting an ingredient "crosswise," is the exact opposite and means slicing across the short way.