Pat the 2½ pounds chicken thighs dry. Season both sides with 1 teaspoon salt, ½ teaspoon black pepper, 1 teaspoon garlic powder, and 1 teaspoon Italian seasoning.
Heat 2 tablespoons olive oil in a large skillet over medium high heat. Add the chicken thighs and sear for 4 to 5 minutes per side until golden brown. The chicken does not need to be fully cooked at this stage. Remove from the skillet and set aside.
Reduce the heat to medium and add 2 tablespoons unsalted butter to the same skillet. Add 2 thinly sliced yellow onions and cook for 10 to 12 minutes, stirring occasionally, until softened and lightly caramelized.
Add 8 ounces sliced cremini mushrooms and cook for 5 to 6 minutes until browned and tender.
Stir in 3 cloves minced garlic and ½ cup chopped sun dried tomatoes. Cook for 30 seconds until fragrant.
Pour in ½ cup dry white wine and scrape up any browned bits from the bottom of the pan. Simmer for 2 to 3 minutes until slightly reduced. Stir in 1 teaspoon Dijon mustard.
Reduce the heat to medium low. Add 1 cup evaporated milk and 8 ounces cubed cream cheese. Stir gently until smooth and fully melted.
Stir in ½ cup grated Parmesan cheese and ¼ teaspoon red pepper flakes, if using.
Add 2 cups fresh spinach and cook until wilted.
Return the chicken thighs to the skillet, nestling them into the sauce. Simmer gently for 5 to 7 minutes, or until the chicken is fully cooked and the sauce has thickened.
Garnish with 2 tablespoons chopped fresh parsley before serving.