Heat olive oil in a large pot.
Add onions and cook until soft.
Stir in garlic and cook briefly.
Add sliced mushrooms and sauté gently.
Place chicken breasts into the pot.
Pour in chicken broth slowly.
Add salt, pepper, thyme, and bay leaf.
Bring soup to a gentle boil.
Reduce heat and simmer 20 minutes.
Remove chicken and shred finely.
Return chicken to the pot.
Simmer five more minutes.
Garnish with fresh parsley before serving.