Heat milk and butter in a small saucepan over medium heat until butter melts, about 2 to 3 minutes. Remove from heat.

Combine flour, baking powder, parsley, and salt in a bowl. Add milk mixture and club soda; stir until dough forms a ball. Cover and let dough stand for 30 minutes.

Bring chicken stock and condensed soup to a simmer over medium-high heat. Add chicken, potatoes, onion, celery, carrot, bell pepper, peas, lemon slices, and thyme. Poach until chicken is no longer pink and vegetables are tender, about 12 minutes.

Transfer chicken to a plate to cool. Transfer vegetables to a bowl, leaving stock in the pan. Discard thyme and lemon.

Boil the remaining stock over medium-high heat until reduced by one-third, about 5 to 7 minutes.

Pour cream into a saucepan and heat over medium-high heat until reduced by about half, about 7 to 10 minutes. Whisk the reduced cream into the stock.

Chop cooled chicken into bite-sized pieces. Melt butter in a skillet and sauté mushrooms with salt and pepper for 2 to 3 minutes. Stir into stock mixture, adding cooked chicken and vegetables.

Bring soup to a simmer over medium heat. Drop teaspoonfuls of dumpling dough into the soup. Cover and simmer for 7 to 8 minutes.