Cook Spinach: Heat 1 Tbsp oil in a large saute pan over medium heat, then add onion. Cook until onion softens (about 3 minutes) then add spinach and cook until it wilts down completely. Drain mixture in a sieve to remove as much moisture as possible.
Make Filling: Preheat the oven to 425°F (220°C). Stir feta, 1 egg, pine nuts, salt, and pepper into spinach mixture.
Assemble: Open your phyllo dough pack and cut pieces into roughly 3 by 10 inch strips, then immediately cover with a layer of plastic wrap and then a damp cloth. Take a piece of phyllo and brush it with olive oil. Place a spoonful of filling at the bottom right of the dough strip. Fold the corner of the dough over to enclose the spinach filling and to form a triangle. Continue folding, maintaining the triangle shape, using the whole dough strip. Repeat to use the rest of the ingredients.
Bake: Place spanakopita triangles on a parchment paper-lined baking sheet and brush with 1 beaten egg. Bake for 25 minutes, or until golden brown.