Heat an 8-quart saucepan over medium heat. Add the olive oil, onion, and garlic, and sweat for 2 minutes until translucent. Add the spinach and cook until it has wilted and is tender, about 4 minutes. Remove from heat and let cool.
Fold in the feta cheese and pine nuts and season with salt. Split hoagie buns and fill with spinach mixture. Wrap the sandwiches in aluminum foil and bake in the oven for 10 minutes.