For the meatballs:

1. To make meatballs, combine chicken breast, bread, onion, milk, mustard, thyme, salt, and pepper in a large bowl. Toss with a fork until combined and smooth, but try not to over-mix. Using your hands, or a small scoop, form the mixture into 1 1/2″ meatballs (I usually get 20-24).
2. Heat a drizzle of olive oil in a large saute pan over medium heat. Working in batches, so as not to overcrowd them, add meatballs to the pan. Cook until well browned and cooked through, turning regularly and adding more olive oil as necessary (this took me about 8-10 minutes per batch). Transfer cooked meatballs to a plate and repeat with remaining meatballs.
3. Add the mushrooms and butter to the same saute pan and cook over medium heat, turning often, until mushrooms are golden and cooked through. If mushrooms begin to stick, feel free to add a little more butter or a small drizzle of olive oil. Add garlic and a pinch of salt and pepper and cook 1 minute more. Sprinkle flour over the mushroom/garlic mixture and stir to coat well. Pour in chicken broth and stir to scrape up any browned bits from the pan. Cook over medium heat 5-7 minutes, or until sauce has thickened somewhat.
4. Whisk in sour cream and Dijon mustard until smooth. Taste sauce and add additional salt and pepper as desired.
5. If serving to children or for ease of serving, slice meatballs in half. Return meatballs to the skillet and stir to coat with sauce. Add noodles to the pan and stir to coat. Top with an additional sprinkle of fresh thyme for garnish, if desired.