Sauté spring onion and sweet onion until soft. Add spinach (in batches), salt, and pepper and sauté until spinach wilts. Bring to a soft boil until most of the water from the spinach has evaporated. Let cool.
Crumble feta into the cooled spinach mixture.
Take a phyllo sheet and spread it out (keep others covered with a towel). Drizzle sheet with olive oil. Cut vertically into 4 strips (long-way - refer to photo below).
Add 1 teaspoon of filling at the bottom of each strip. Fold once away from you, then continue folding diagonally into small triangles (see photos below). Continue until entire sheet is folded. Repeat for each sheet of phyllo, until the mixture is done.
Lay triangles on a baking sheet. Optional: Whisk 1 egg yolk in a bowl, and brush on the tops of each triangle.