In a large mixing bowl, combine the buckwheat flour and salt.
Gradually add water while mixing until a smooth and elastic dough forms.
Knead the dough for about 5 minutes, then cover it with a damp cloth and let it rest for at least 30 minutes.

In a skillet, heat the vegetable oil over medium heat.
Add the finely chopped onion and minced garlic, and sauté until the onion is soft and translucent.
Add the chopped spinach or Swiss chard and cook until wilted.
Remove from heat and let it cool slightly.
In a mixing bowl, combine the cooked spinach mixture with grated cheese, soy sauce, ground black pepper, and salt to taste. Mix well.

Divide the dough into small, equal-sized balls.
Roll each ball into a thin circle on a lightly floured surface.
Place a spoonful of the filling in the center of each circle.
Fold the dough over the filling to form a half-moon shape and press the edges to seal. You can crimp the edges with a fork for a decorative touch.

In a large skillet, heat the vegetable oil over medium heat.
Arrange the dumplings in the skillet and cook until the bottoms are golden brown, about 2-3 minutes.
Carefully pour in the water and cover the skillet with a lid.
Let the dumplings steam for about 5-7 minutes, or until the water has evaporated and the dumplings are cooked through.

Transfer the hoentay to a serving plate.
Serve hot, with a dipping sauce of your choice.
"Enjoy your meal!"