Prepare the filling by sautéing the onions in olive oil.
Add the spinach to the skillet, cooking until wilted.
Mix in the feta, dill, salt, and pepper.
Preheat your oven to 375°F (190°C) and prepare a baking sheet.
Unroll the phyllo dough and cover with a damp cloth.
Lay one sheet of phyllo on a clean surface and brush with melted butter.
Fold the phyllo dough in half lengthwise.
Place a spoonful of filling at one end, then fold into a triangle.
Repeat the process with remaining ingredients and place on the baking sheet.
Brush the tops of the triangles with more melted butter before baking.
Bake in the preheated oven for 20-25 minutes or until golden brown.