Set a 12-inch skillet with a lid over medium heat. Let it heat for about 1 minute so the vegetables start cooking right away.

Add the oil to the skillet. When the oil looks loose and shiny, add the chopped onion and the white parts of the scallions.

Cook the onion mixture for 3 to 4 minutes, stirring often, until the onion softens and starts to turn light gold at the edges.

Add the sliced mushrooms to the skillet and spread them into an even layer. Cook for 5 to 6 minutes, stirring only once or twice, until they give off their moisture and brown in spots.

Season the vegetables lightly with salt and black pepper. Stir well and keep the skillet over medium heat.

Add the rinsed rice to the skillet. Stir for 1 minute, until the grains look glossy and smell lightly toasted.

Put the miso and soy sauce in a measuring cup or bowl. Whisk in 2 cups water until the miso is mostly dissolved.

Pour the miso mixture into the skillet. Scrape the bottom of the skillet with a spoon to loosen any browned bits, then bring the liquid to a gentle boil.

When the liquid is bubbling, cover the skillet with the lid and turn the heat to low. Cook for 15 minutes without lifting the lid.

Remove the lid and scatter the edamame over the rice. Cover the skillet again and cook for 5 minutes more over low heat.

Lift the lid and check the rice near the center of the skillet. If the grains still look firm or the skillet looks dry before the rice is tender, add 1/4 cup water, cover, and cook for 2 to 3 minutes more.

Add the spinach to the skillet in two handfuls, placing it directly on top of the hot rice. Cover the skillet for 1 minute so the spinach can wilt.