Shiitake Mushroom Yakitori

1.
Make the sauce: Put the soy sauce, mirin, and sugar into a small saucepan and cook over medium-low heat.
When the mixture begins to boil, reduce the heat to low, and continue cooking over low heat for 20 minutes.
Skim any scum off the surface as the sauce is cooking.
Set aside.
2.
Make the skewers: Soak the bamboo skewers in water for 20 minutes.
Preheat the grill.
Clean the mushrooms with a slightly damp paper towel or cotton cloth, then cut away and discard the stems.
Cut the firm white and whitish green parts of the scallions into 1 3/4 inch lengths.
3.
Thread two mushrooms (lengthwise through the mushroom caps) and two pieces of scallion onto each skewer, alternating between the mushrooms and scallion pieces.
Brush the mushrooms and scallions with a light layer of vegetable oil.
4.
Place the skewers on the grill, with mushrooms facing gill side up.
Cook the skewers until the tops of the mushroom caps are dry.
Turn the skewers over (mushrooms gill side down), and cook them until the insides become wet with the mushrooms own juice.
Turn the skewers over (mushrooms gill side up) one more time and cook for about 1 or 2 minutes more until the mushrooms and scallions are completely cooked through.
5.
Remove the skewers from the grill, and with a pastry brush, baste them with the tare sauce.
Arrange the skewers on a large plate and serve.