Heat a large skillet over medium-high heat and add the olive oil.
Pat the chicken dry with paper towels. In a small bowl, mix together the Italian seasoning, paprika, onion powder, garlic powder, salt, pepper, and red pepper flakes.
Season both sides of the chicken generously with the spice mixture.
Sear the chicken for 2–3 minutes per side until lightly golden. The chicken does not need to be fully cooked through.
Transfer the chicken to the crockpot along with any leftover juices from the skillet.
Add the garlic, sun-dried tomatoes, mushrooms, and tomato paste around and over the chicken.
In a medium bowl, whisk together the arrowroot starch, chicken broth, and Worcestershire sauce until smooth.
Pour the broth mixture into the crockpot around the chicken.
Cover and cook: LOW for 3½–4 hours for chicken breasts, LOW for 4½–5 hours for chicken thighs
During the last 10–15 minutes of cooking, stir in the heavy cream, parmesan cheese, and spinach.
Cover again and cook until the spinach wilts and the sauce becomes creamy and smooth.
Finish with fresh lemon juice right before serving. Taste and adjust salt if needed.
Garnish with fresh basil or parsley and serve warm over pasta.