In a large pot or Dutch oven, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until soft and translucent.
Stir in the garlic and sliced mushrooms, cooking for an additional 5 minutes until the mushrooms are tender and have released their moisture.
Add the chicken thighs to the pot, along with the chicken broth, water, wild rice, thyme, rosemary, salt, and pepper. Bring the mixture to a boil.
Once boiling, reduce the heat to low, cover, and let it simmer for about 30 minutes, or until the wild rice is cooked and the chicken is tender.
Remove the chicken thighs from the pot and shred them with two forks. Return the shredded chicken to the soup.
Stir in the heavy cream and fresh spinach (if using), simmer for an additional 5 minutes, until the spinach has wilted and the soup is heated through.
Taste and adjust seasoning as necessary. Serve hot, garnished with fresh parsley if desired.