Transfer chicken thighs to a clean plate and pat dry with a paper towel. Generously season both sides with salt, black pepper, and Italian seasoning.
Heat the olive oil in a large skillet over medium-high heat. Add the chicken thighs, skin-side down, and sear until golden brown, approximately 3 minutes. Turn over and sear the other side for 3 minutes. Reduce heat to medium and cover the pan with a tight-fitting lid.
Continue to cook the chicken until the internal temperature registers 160°F as measured with a digital meat thermometer, approximately 15 minutes.
Remove chicken to a clean plate and set aside.
Return the skillet to medium heat. Add the onion and cook, stirring occasionally, for 2-3 minutes or until just starting to soften. Add the garlic and fresh thyme. Continue to cook for 1 minute more.
Add the chicken broth, sundried tomato pesto, roughly chopped sundried tomatoes, and fresh spinach. Mix well to combine with the minced onions. Add the heavy cream and bring to a simmer. Sprinkle with parmesan cheese and salt, to taste.
Nestle the cooked chicken thighs into the simmering cream sauce. Cook everything, all together, for 2 minutes, or until chicken is heated through.
Serve with your favorite sides (like quinoa, cooked pasta, or farro) and garnish with fresh thyme and red chili flakes, if desired.