Step 1: Preheat a large heavy-bottomed pot or Dutch oven over medium heat. Add butter and let it melt completely. Add chopped onion and minced garlic. Sauté for 3-4 minutes until onion becomes translucent.
Step 2: Add carrots, celery, fresh mushrooms (if using), thyme, garlic powder, salt, and pepper. Sauté for 5 minutes, stirring occasionally.
Step 3: Add chicken pieces, diced potatoes, dried mushrooms (if using), and wild rice. Pour in 7 cups of water or chicken broth. Stir to combine all ingredients.
Step 4: Cover the pot and bring to a boil. Reduce heat to low and simmer for 55 minutes, or until rice and potatoes are tender.
Step 5: Remove chicken pieces from pot using tongs. Place on a plate and shred with 2 forks. Set aside.
Step 6: Add milk and prepared mustard to the pot. Use an immersion blender to pulse 3-5 times until desired consistency (or transfer 2 cups at a time to a regular blender).
Step 7: Return shredded chicken to pot. Stir in fresh chopped parsley. Taste and adjust seasonings if needed. Let sit for 5 minutes before serving.