To begin, bring a large pot of salted water to a boil.
Then, get started on the sauce. Add olive oil to a large saute pan or dutch oven and bring to medium heat.
Season the cubed chicken and add it to the pan. Saute until the chicken is slightly browned and then remove it and set aside.
If the saute pan is dry after removing the chicken, add a couple of more drizzles of olive oil. Then, add the chopped onion and saute until soft, about 3-4 minutes.
Deglaze the pan with white wine, scraping up the brown bits that stuck to the bottom of the pan. Allow the onions to simmer in the wine for about 5 minutes, until the alcohol burns off.
Then, add the stoneground mustard and stir. It will look a little lumpy, but it will dissolve as it continues to cook.
Next, add the chicken stock and heavy cream. Mix to combine.
Add the thyme, salt, and pepper. Bring the sauce to a boil, and then reduce to a simmer. Taste to adjust seasoning.
Return the chicken and its juices to the pan and cook over medium high heat for about 5 minutes or until the chicken is cooked through. Reduce the heat to low. Add the spinach, stir and allow the sauce to simmer while you cook the pasta.
Cook the pasta to al dente, per the package instructions and then transfer the pasta to the sauce to finish cooking. At this point, tailor the sauce to your desired consistency. To thin the sauce, add a small amount of pasta water at a time and stir. Alternately, you could thicken the sauce by transferring 1/2 cup of the sauce into a heat safe bowl and adding 1 Tbspn of cornstarch. Whisk until the lumps have dissolved and add it back to the sauce. Stir and the sauce will thicken within minutes.Top with chopped fresh spinach for garnish and serve.