In a medium pan, heat oil over medium-high. Add onion and cook until golden, about 4 minutes. Add garlic and cook until fragrant, 1 minute. Transfer to a large bowl.
Return pan to heat. Working in batches, cook spinach, tossing, until wilted, about 1 minute. Drain spinach and let cool. Press out excess moisture, then roughly chop. Add spinach, feta, and raisins to bowl with onion and season filling with salt and pepper. (To store, refrigerate, up to 2 days.)
Preheat oven to 350 degrees. Unroll phyllo. Place 1 sheet on a work surface (keep remaining sheets covered with a damp kitchen towel to prevent drying); brush with oil. Lay another sheet on top and brush with oil. Repeat until you have 10 layers of phyllo. Spread half of filling lengthwise down middle of phyllo stack. Roll into a log, brushing the seam with oil before sealing. Brush log with oil. Using a small sharp knife, make 8 diagonal cuts on top. Repeat with remaining phyllo and filling.
Place logs, seam side down, on a parchment-lined baking sheet. Bake until deep golden brown, 45 to 50 minutes. Cut into slices along slashes. Serve warm or at room temperature.
When you unroll your phyllo, check to see if the stack measures 8 by 12 inches. If not, cut the phyllo to size. If serving the pie as a main course, double the recipe.