Heat oil over medium heat. When hot, add the sausage. As it cooks, gradually turn up the heat to medium-high so the moisture cooks off immediately and the sausages get crispy. Remove from pan and set aside.
Lower heat to medium. Add onion to the pan. Cook for 2-3 minutes, then add garlic and cook until both are softened.
Push the garlic and onions aside in the pan. Gradually whisk the arrowroot into the oil in the pan. Stir the onion and garlic into the mixture to coat everything.
Slowly whisk the broth into the pan, scraping up the brown bits as you go. Let it boil and thicken in between adding broth.
Slowly whisk in the milk. Whisk in the mustard and lemon juice.
Add in the sausages, spinach, and sun dried tomatoes. When everything has heated through, let it simmer a few minutes to thicken if needed. If using the parmesan cheese, add it now and stir.
Serve together and enjoy!