In a skillet over medium heat, melt 1 tablespoon of butter. Add the finely chopped onions and sauté until they are translucent and slightly golden, about 5 minutes. Transfer the cooked onions to a mixing bowl.
To the mixing bowl with onions, add the drained cottage cheese, thawed and squeezed dry spinach, beaten egg, crumbled feta, and ground black pepper. Mix well until all ingredients are fully combined.
Lay out one sheet of defrosted phyllo dough on a clean surface. Brush it lightly with melted butter. Place another sheet of phyllo dough on top, and brush it again with butter. Repeat this process until you have layered three sheets of phyllo.
Using a sharp knife or pizza cutter, cut the layered phyllo dough into long strips, about 3 inches wide.
At the end of each strip, place a spoonful of the spinach filling. Fold the corner of the strip over the filling to form a triangle. Continue folding the triangle along the strip until you reach the end. Seal the edge with a little melted butter.
Preheat your oven to 375°F (190°C). Arrange the filled triangles on a parchment-lined baking sheet. Brush the tops with more melted butter to ensure they become golden and crispy during baking. Bake for 20-25 minutes or until they are golden brown.
