Prepare Haminados Eggs: Place eggs in a pot, add oil, salt, pepper, onion peels, and coffee grounds (if using).

Cover with water and bring to a boil, then simmer for at least 1 hour, or overnight on very low heat.

Prepare Potato Filling: Fry thinly sliced onion in olive oil until golden.

Boil and mash potatoes. Season with salt and pepper.

Mix fried onions and crumbled feta cheese (optional) with mashed potatoes.

Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.

Roll puff pastry thinly (about 1/4 inch) and cut into 6 equal squares.

Divide potato filling into 6 portions.

Place one portion in the center of each pastry square. Fold into a triangle and seal the edges.

Brush each triangle with beaten egg yolk and water mixture.

Sprinkle with sesame seeds.

Bake on the middle rack of the preheated oven for 25-30 minutes, or until golden brown and crisp. Ensure the bottoms are cooked.

Serve warm or at room temperature with Haminados Eggs.