Preheat oven to 350 degrees F.

Heat an 8-quart saucepan over medium heat.

Add the olive oil, onion, and garlic to the saucepan.

Sweat the onion and garlic for 2 minutes until translucent.

Add the spinach to the saucepan.

Cook the spinach until it has wilted and is tender, about 4 minutes.

Remove the saucepan from heat and let the spinach mixture cool.

Fold in the feta cheese and pine nuts into the cooled spinach mixture.

Season the spinach mixture with salt to taste.

Split the hoagie buns lengthwise.

Fill each hoagie bun with the spinach mixture.

Wrap the filled sandwiches in aluminum foil.

Bake the wrapped sandwiches in the preheated oven for 10 minutes.

Unwrap the sandwiches and serve immediately.