Season chicken with salt and pepper. Heat olive oil in a large skillet over medium-high heat and cook chicken until browned on both sides, about 10 minutes. Transfer to slow cooker.
Add butter, garlic, sun-dried tomatoes and chicken stock.
Cover and cook on low for 6 hours or high for 2 hours. Transfer chicken to a plate.
Place flour in a medium bowl and carefully mix in a few tablespoons of hot cooking liquid. Whisk smooth and return the slurry to the slow cooker. Stir in heavy cream and simmer for 3- 5 minutes, or until thickened.
Stir in parmesan and spinach and cook until wilted. Return chicken to pot and coat with sauce. Serve immediately.
Instant Pot directions: Use Instant Pot’s “Sauté” setting to brown the chicken according to Step 1. Add everything in Step 2 to the pot, then secure the lid and close the pressure valve. Cook on manual for 10 minutes at high pressure. After the timer goes off, use a quick release to release the pressure. Remove the lid and transfer chicken to a plate. Continue with the recipe in step 4.