Heat 1 tablespoon of oil and 1 tablespoon of butter in a Dutch oven over medium high heat for 2 minutes. Add half the mushrooms in a single layer and cook 4 to 5 minutes without stirring, until deep golden, then stir and cook 2 minutes more, and move them to a bowl. Repeat with the remaining oil and mushrooms.

Melt the remaining 1 tablespoon of butter in the same pot over medium heat. Cook the onion for 4 minutes, until soft and translucent, then add the garlic and thyme for 30 seconds, until fragrant.

Sprinkle the flour over the onion and stir for 1 minute, until it smells nutty and no dry flour remains.

Pour in the broth 1 cup at a time, whisking until smooth, and scrape up the browned bits from the bottom. Add the bay leaf, half the seared mushrooms, the salt, and the pepper, then simmer for 12 minutes, until the soup coats the back of a spoon.

Remove the bay leaf, then stir in the milk and cream. Warm the soup over low heat for 3 minutes, without letting it boil, until steaming.

Stir in the chicken, the remaining mushrooms, and the spinach for 2 minutes, until the spinach wilts and the chicken reaches 165°F. Stir in the lemon juice and taste for salt.

Ladle the soup into warm bowls, top with a thyme sprig and a crack of black pepper, and serve right away with crusty bread.