Prep the crockpot: Lightly grease the insert or use a liner. Add the diced onion and garlic to the bottom.
Add chicken and seasonings: Place the chicken on top.
Sprinkle with Italian seasoning, red pepper flakes (if using), salt, and black pepper. Scatter the sliced sun-dried tomatoes over the chicken. Drizzle 1–2 tablespoons of the sun-dried tomato oil over everything for extra flavor.
Pour in broth: Add the chicken broth around the sides so it distributes evenly.
Cover the crockpot.
Cook: Cook on Low for 3.5–5 hours or High for 2–3 hours, until the chicken is tender and reaches 165°F internally. Thighs tend to be more forgiving and can go a bit longer without drying.
Shred or slice chicken: Transfer the cooked chicken to a plate. Shred with two forks or slice into thick pieces.
Keep warm.
Make it creamy: To the hot liquid in the crockpot, add cream cheese cubes and heavy cream. Whisk until smooth. Stir in Parmesan until melted.
If needed, cover and let the residual heat soften the cream cheese for a few minutes before whisking again.
Add spinach: Stir in the spinach. Cover for 2–3 minutes until wilted.
Finish the sauce: Return the chicken to the crockpot and stir to coat. Add lemon juice if using.
Taste and adjust with more salt, pepper, or Parmesan as needed.
Serve: Spoon over pasta, rice, mashed potatoes, polenta, or roasted vegetables. Garnish with fresh basil.