Start the broth: Put a large soup pot on the stove over medium heat. Add the neutral oil and let it warm for about 30 seconds.

Add the sliced cremini mushrooms and the scallion whites to the pot. Cook for 5 to 6 minutes, stirring every minute, until the mushrooms shrink and start to brown around the edges.

Add the grated ginger to the pot. Stir for about 30 seconds, just until it smells fragrant.

Pour the chicken broth into the pot. Add the dried shiitake mushrooms to the broth.

Raise the heat to high and bring the broth to a boil.

Lower the heat to medium-low and let the broth simmer for 8 minutes. The dried shiitake should soften and look plump.

Mix the miso: Put the white miso in a small bowl.

Ladle about 1/2 cup of the hot broth from the pot into the bowl with the miso. Whisk with a fork until the miso is smooth and mostly dissolved, then set the bowl aside.

Use tongs to move the softened dried shiitake mushrooms from the pot to a cutting board. Slice them thinly, then put the sliced shiitake back into the pot.

Finish the soup: Add the shredded rotisserie chicken to the pot. Increase the heat to medium and let the broth come back to a gentle boil, about 2 minutes.

Put the frozen udon noodles into the pot. Cook for 3 to 4 minutes, using tongs or a spoon to loosen the noodles as they heat, until the strands separate and are tender.

Add the baby spinach to the pot. Stir for 1 minute, until the spinach wilts into the broth.