Warm the olive oil in a large Dutch oven or stock pot over medium heat. Add the onions and cook until translucent, about 5 minutes.

Add the mushrooms and sprinkle with 1/2 teaspoon of salt. Cook until they have released their liquid and most of the liquid has evaporated, about 10 minutes.

Clear a little space in the middle of the pan and add the garlic. Cook until fragrant, 30 to 60 seconds, and then stir into the vegetables.

Add the chicken stock, water, thyme, bay leaf, and the remaining 1 1/2 teaspoons of salt. Increase the heat to high and bring to a boil. Add the chicken and let the soup return to a rapid simmer. Reduce the heat to medium-low and cook until the chicken is cooked through, about 10 to 15 minutes.

Transfer the cooked chicken to a plate to cool enough to handle.

Bring the soup back to a rapid simmer. Break the lasagna noodles into bite-sized pieces and add to the soup. Stir in the baby spinach and cook until the noodles are al dente and the spinach is wilted, about 8 to 10 minutes.

Shred the chicken into small pieces with two forks.

When the noodles and spinach are done, add the shredded chicken back to the soup, remove the thyme stems and bay leaf. Stir in the shredded Parmesan and the milk, and return the soup to a simmer. Taste the broth and add more salt or pepper if needed.

Remove from the heat and ladle into bowls. Serve with a dollop or two of ricotta and freshly ground black pepper.