In a large bowl, combine the flour, water, salt, and egg. Knead the mixture until it forms a smooth, elastic dough. Cover the dough with a damp cloth and let it rest for about 45 minutes to 1 hour.

In a separate bowl, mix the ground beef, grated onion, black pepper, and salt. Set aside.

Roll the rested dough out on a lightly floured surface to about ⅛ inch thick. Cut it into ½-inch squares. Place a small amount of the beef filling in the center of each square. Fold the corners, meeting at the center, pinching the edges to form pouch-like small dumplings. Repeat the process until all ingredients are used.

Bring a large pot of salted water to a boil. Add the manti and cook for 6-7 minutes or until they float to the surface. Remove with a slotted spoon and drain.

In a small bowl, whisk together the Greek yogurt, water, minced garlic, and salt until smooth. Adjust the water as needed to get a pourable consistency. Set aside.

In a small saucepan, melt the butter over medium heat. Add the dried mint and red pepper flakes, stirring until fragrant.

Serve the cooked manti on a plate, drizzle with the yogurt topping, and spoon the butter sauce over the top. Enjoy!