Heat the olive oil in a large skillet over medium-high heat and add chicken. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Saute until chicken is no longer pink outside, about 5 minutes. Remove chicken from pan and set aside.
In a large heavy pot, heat the butter over medium heat. Add the mushrooms, leek, bell pepper and carrots and cook, stirring often, until softened (about 5 minutes). Stir in 1/8 teaspoon salt, 1/8 teaspoon pepper and garlic, and cook for 1-2 minutes, stirring constantly. Add white wine vinegar and thyme and cook 1 minutes more. Add flour to the vegetables, stirring to coat them all, and cook for 1 minute longer.
Add chicken broth and water, and simmer slowly for 15 minutes. Add spinach leaves and chicken and continue simmering for another 15 minutes. Stir in heavy cream, simmer for 3-5 minutes. Taste and add more salt or pepper, if desired.
Ladle soup into bowls and top with Parmesan cheese and croutons. Enjoy!