Season the chicken thighs evenly with salt, black pepper, garlic powder, and onion powder on both sides.
Heat olive oil in a large skillet over medium-high heat. Add the chicken thighs skin-side down and sear until golden brown, about 4 minutes per side. Remove chicken and set aside.
Add sliced onion to the same skillet and sauté for 3-4 minutes until softened. Add minced garlic and cook for 30 seconds until fragrant.
Transfer the sautéed onions and garlic to the slow cooker.
Place the seared chicken thighs on top of the onions in the slow cooker.
Pour chicken broth over the chicken and sprinkle dried thyme and paprika evenly.
Cover and cook on low for 5 to 6 hours, or until the chicken is tender and cooked through.
About 30 minutes before serving, melt butter in a small saucepan over medium heat. Whisk in flour and cook for 1-2 minutes to form a roux.
Slowly whisk in the heavy cream and 1/2 cup of the cooking liquid from the slow cooker. Continue whisking until the gravy thickens, about 3-5 minutes.
Pour the thickened gravy back into the slow cooker and stir gently to combine with the chicken and juices.
Serve the chicken with the creamy gravy spooned over the top.