quick and easy gluten-free oyakodon (Japanese chicken and egg dish)

I've been making oyakodon, a Japanese chicken and egg dish to put on top of a bowl of rice, for years and years. The recipe I had was a bit complicated and time-consuming, so I was happy when a friend here in Japan passed along a simpler version. I can't remember where it's from exactly, and I've changed it a little bit anyway, so here's my take. This makes enough for four people. 

You'll need: 

• 2 chicken breasts, chopped in bite-sized pieces
• 1 bunch of spinach, washed and chopped
• any kind of mushrooms, chopped
• 2 onions, sliced
• 6 eggs, beaten
• oil for cooking
• 4 tablespoons cooking sake
• 1 tablespoon sugar
• 1/4 cup homemade dashi (with konbu and katsuobushi flakes)
• 4 tablespoons gluten-free soy sauce

Do this:

Heat up your oil in a frying pan and cook the chicken till all the pieces are browned. Add the mushrooms and onions, then all the liquids and sugar. When everything is soft, add the spinach for a minute and then dump the eggs over it all and let it set till cooked. 

Serve over rice in a bowl, and add all those delicious juices and then eat with a spoon.