Heat oven to 375°F. Parchment two sheet trays and set aside. Add olive oil and onion to a large skillet over medium heat and simmer for 10 minutes, stirring regularly.
Step 1
Garlic should be aromatic after 30 seconds. Add spinach and simmer until wilted, possibly adding it slowly. While stirring occasionally, cook until the liquid is largely gone, 10-15 minutes.
Step 2
Place on platter until room-temperature. Stir together chilled spinach, feta, parmesan, egg, dill, salt, and pepper in a large bowl.
Step 3
Phyllo dough should be placed on a sheet tray with a damp kitchen towel on top to prevent drying. Put one phyllo dough sheet on a clean surface. Lightly brush melted butter.
Step 4
Put another sheet on top and butter it. Repeat twice more, stacking four butter-brushed sheets. Trim the phyllo into three long, even strips.
Step 5
Put a heaping scoop of filling in the bottom right corner of each phyllo strip. Invert the filling from the bottom right corner of the phyllo strip to face the left straight edge.
Step 6
Bottom of strip will look like a triangle. Fold up again on the other triangular side. Repeat flipping the triangle until the strip is used. Put the triangles on a sheet tray.
Step 7
Use the leftover phyllo sheets and filling. Bake triangles until golden brown, brushing with remaining butter. Serve mildly chilled.