Place the cubed chicken in a medium-sized bowl and add 1/2 teaspoon of black pepper and 1 1/2 teaspoons of kosher salt for seasoning. To coat, mix.
In a Dutch oven set over medium-high heat, melt 1 tablespoon of butter. Add the chicken and sear it for 5–6 minutes on one side. For an additional five minutes, stir and sear the opposite side. Take out of the pan and place aside.
Reduce the heat to medium and melt two tablespoons of butter. Use a spoon to scrape up some of the flavorful browned crumbs from the pan's bottom as it melts. To make a roux, add two teaspoons of flour and stir. Stir constantly until a roux forms.
Add the onion and garlic, cover with roux, and cook, stirring often, for one to two minutes. Saute for a further two to three minutes after adding the carrots, celery, celery salt, ground mustard, sage, oregano, and 1/4 teaspoon of black pepper.
Increase the heat to medium-high and gradually add the chicken stock, stirring and scraping off any browned pieces from the pan's bottom as you go. Add 1 cup of milk after adding all of the stock.
Bring the chicken back to a boil in the Dutch oven before lowering the heat to a simmer. Pour in the lemon juice. Simmer for ten minutes.
In the meantime, transfer flour and baking soda into a medium-sized basin by sieving. Add and stir in salt. Two tablespoons of cubed, cold butter should be added. Using a pastry cutter, chop butter into flour until it is completely combined and the dough is crumbly.
Mix in the dry parsley and then the milk, continuing to mix until a sticky batter or dough forms. While the soup simmers, set aside.
Simmer for ten minutes, then increase the heat to a soft boil once more. Gather spoonfuls of dough and carefully drop them into the soup using an oil-coated spoon or dough scooper (optional but will assist prevent sticking). Continue separating them with some space until all of the dough has been utilized, which should yield 6–8 dumplings.
To allow the dumplings simmer and the soup thicken, cover and boil for an additional fifteen minutes.
Reduce heat after 15 minutes, serve, and savor!