Combine water and salt in a small saucepan over medium heat and warm through, stirring until salt is dissolved. Remove from heat and let cool for 2 to 3 minutes.

Combine sifted flour and melted margarine in the bowl of a food processor; pulse until crumbly. Add salt water gradually to the food processor and pulse until a soft dough ball forms that easily separates from the edge of the bowl. Add more water, 1 teaspoon at a time, only if needed. Press dough into a ball and wrap tightly with plastic wrap. Refrigerate for at least 30 minutes.

Preheat the oven to 425°F (220°C). Cover a baking sheet with aluminum foil and grease with vegetable oil. Set aside.

Bring a saucepan of water to a boil. Add cubed potatoes and cook until soft, 8 to 10 minutes.

Meanwhile, melt butter in a skillet over medium heat. Cook and stir onions and green onions until soft and translucent, about 5 minutes. Add red peppers and cook for 5 more minutes. Add ground beef and stir with a wooden spoon until fully browned, about 5 minutes. Season with salt, cayenne pepper, cumin, and paprika. Remove from heat.

Drain potatoes and add to filling. Toss in chopped eggs and let filling cool down for a few minutes.

Remove pastry from the fridge and knead gently on a floured surface. Divide dough in half and roll out each piece to 1/8-inch thickness.

Cut out 3-inch circles with a pastry cutter or sharp knife and wet the edges lightly with water. Add 1 large tablespoon of filling in the center of each disc. Fold pastry over, press edges together, and seal with a fork. Transfer empanadas to the prepared baking sheet.