Combine flour and salt in a large bowl. Add eggs and water, mixing well with your hands. Add more water, if needed, to form a soft dough. Cover and set aside for at least 30 minutes.

Shred onions and place them in a colander or sieve set over a bowl; drain and discard juice. Combine drained onion, ground beef, salt, and pepper in a medium bowl; mix well with a spoon until mashed. Set filling aside.

Lightly flour a work surface and a large plate.

Divide dough in half. Working with one piece at a time, roll dough on the floured surface into a rectangle as thin as possible. Cut into 2-inch squares using a knife or pastry wheel.

Place about 2 teaspoons filling in the center of each square. Gather the edges of dough and pinch them together at the top to form a bundle. Transfer manti to the prepared plate; sprinkle more flour on top to prevent sticking.

Heat vegetable oil and red pepper flakes in a small skillet over low heat just until pepper flakes begin to color the oil. Remove from heat and keep warm.

Stir together yogurt and garlic in a small bowl; set aside.

Bring a large pot of salted water to a boil over medium-high heat. Cook manti in boiling water until filling is no longer pink and dough is tender, 20 to 25 minutes. Drain well.