Thaw Phyllo Dough: Place the frozen phyllo dough in the fridge overnight.
Preheat oven: 350°F or 180°C
Bake Spinach: Bake the frozen spinach in the oven for about 10 minutes or until completely thawed. Let it cool. Then using a colander, remove as much excess liquid from the spinach as possible.
Saute leek and onion: While the spinach mixture is baking, heat oil in a large skillet over medium heat. Saute the leek and onion until soft and lightly browned. Remove from the heat, add to spinach, and set aside to cool.
Combine the Filling: In a medium bowl, mix in feta cheese, chopped dill, nutmeg, and spinach mixture.
Assemble: Open your phyllo dough pack and using one sheet, lay the short side towards yourself, brush with olive oil. Repeat the process, layering two more times, placing the sheets of phyllo dough over the top of each other (3 sheets in total). Cut the phyllo dough layers into three equal sections longways. Add one tablespoon of the cooled spinach filling at the end closest to you.
Shape your triangles: Starting from the bottom end, fold up into mini spanakopita triangle bites (like pictured) above. Work fast, so your phyllo dough doesn't dry out.
Bake: Place spanakopita triangles on a baking sheet lined with parchment paper. Brush with olive oil and lightly sprinkle with sesame seeds—Bake for 25 minutes, or until golden brown.