PASTA: Set a large colander in the sink with a glass mug or liquid measuring cup in the center of the colander. (This is so you don't forget to reserve some pasta water before draining the pasta!) Bring a large pot of water to a boil. Generously salt the water and then cook the pasta according to package instructions subtracting 1-1/2 minutes from what the package indicates. Use the glass mug to remove a big scoop of pasta water (set aside for now) and then drain the pasta and set aside. Don't rinse under cool water.
SAUSAGE: Meanwhile, add 1 tablespoon olive oil to a large heavy bottom skillet. Heat to medium-high heat and add in the sausage, 1/2 teaspoon Italian seasoning, and salt & pepper to taste (I add 1/2 teaspoon of each). Cook, crumbling with a wooden spoon until browned. Use a slotted spoon to remove the sausage onto a paper-towel-lined plate. Leave any grease behind, but if there is more than 1 tablespoon grease, drain off the excess.
GARLIC: Reduce heat to medium and add the butter to the skillet. Once melted, add garlic and a tiny pinch of salt & pepper. Cook, until golden, about 30 seconds. Pour in 1/2 cup of chicken broth; it should sizzle and boil. Scrape up any browned bits off the bottom of the pan. Once this 1/2 cup is mostly evaporated, add in the other 1/2 cup. Stir.
CREAM SAUCE: Add in the heavy cream, Parmesan cheese, and sun-dried tomatoes. Simmer, over medium heat, stirring occasionally for 3-5 minutes or until it thickens up and slightly reduces. The sauce shouldn't boil; reduce heat/stir more frequently it needed. Once thickened, mix in the remaining 1 teaspoon Italian seasoning and Dijon mustard if using.
TOSS: Add the spinach and pasta and use tongs to toss into the cream sauce until the sauce thickens, nicely coats the pasta, and the spinach cooks down (1-2 mins). If needed (if the sauce is too thick), slowly add in that reserved pasta water. After tossing for about a minute, add in the reserved sausage and continue to toss.
SERVE: Serve immediately. Top individual plates with a touch more Parmesan if desired, and fresh thyme or basil if using. Enjoy immediately -- we love serving some crusty toasted sourdough on the side to sop up extra sauce! Pasta is best enjoyed right after being tossed!