Add cubed chicken to a medium-sized bowl and season with 1 1/2 teaspoons of kosher salt and 1/2 teaspoon of black pepper. Mix to coat.
Melt 1 tablespoon of butter in a dutch oven over medium-high heat and add chicken and sear on one side for 5-6 minutes. Stir and sear on the other side for another 5 minutes. Remove from pan and set aside.
Turn the heat down to medium and add 2 tablespoons of butter to melt. While it melts, use a spoon to scrape up some of the browned bits (flavor) off of the bottom of the pan. Add 2 tablespoons of flour and whisk to create a roux. Whisk continuously until a roux is formed.
Add the onion and garlic and coat with roux, saute for about 1-2 minutes stirring often. Add celery, carrots, celery salt, ground mustard, sage, oregano, and 1/4 teaspoon of black pepper and saute for another 2-3 minutes.
Turn heat up to medium high and slowly add chicken stock, mixing as you add and scraping up any browned bits on the bottom of the pan while adding. Once all of the stock is added, add 1 cup of milk.
Return chicken to the dutch oven and bring to a boil and then reduce to a simmer. Add lemon juice. Let simmer for 10 minutes.
Meanwhile, sift flour and baking soda into a medium-sized bowl. Add salt and mix. Add cubed cold 2 tablespoons of butter and use a pastry cutter to cut butter into flour until it's all incorporated and you have a crumbly mixture.
Add dried parsley and mix, then add milk and mix until a sticky dough / batter is formed. Set aside while soup is simmering.
After 10 minutes of simmering, turn the heat back up to a gentle boil. Spray a spoon or dough scooper with oil (optional but will help reduce dough from sticking) and then gather scoops of dough and gently drop them into the soup. Repeat, giving some space between until all of the dough has been used, about 6-8 dumplings.
Cover and let boil for another 15 miutes to let the dumplings cook and soup thicken.
After 15 minutes, reduce heat, serve and enjoy!