Prepare Gravy Mixture:
In a large spouted measuring cup, whisk the gravy mix with 1 cup cold water until smooth. Stir in the tomato sauce, Italian seasoning, parsley, mustard powder, and 1/4 teaspoon salt. Reserve.Season and Coat Chicken:
Dice the chicken into uniform cubes; season with salt and pepper. Dust with flour and toss to coat evenly.Sear Chicken:
Heat olive oil in a large skillet over medium-high heat. Add chicken and sear for 3-4 minutes until golden brown. Flip and cook the other sides until fully cooked, about 3 more minutes. Remove chicken and set aside.Deglaze Skillet:
Turn off the heat. Pour in 1/4 cup of the wine, then return heat to medium. Using a silicone spatula, deglaze the skillet by scraping up browned bits from the bottom and sides.Caramelize Onions:
Add butter and sliced onion. Cook over medium heat, stirring frequently, until very soft and nearly caramelized, about 20 minutes.Add Garlic:
Add minced garlic and cook for 1 minute.Reduce Wine:
Pour in the remaining wine; let it simmer and reduce for 5 minutes.Simmer Sauce with Chicken:
Add the gravy mixture to the skillet. Bring to a boil and let thicken for 30 seconds, then reduce to a simmer. Return the chicken to the skillet.Cook Spaghetti:
Partially cover the skillet while you bring a pot of water to a boil for the spaghetti. Once boiling, salt the water (about 1/4 tablespoon). Cook spaghetti until al dente, then drain, reserving some pasta water if desired.Toss Spaghetti with Sauce:
Add the cooked spaghetti to the sauce and toss to coat. Garnish with parsley and serve with Garlic Bread with Cheese.