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 for 4-5 minutes until the skin is golden brown. Flip and sear the other side for 3 minutes. Remove chicken and set aside.
In the same skillet, add the sliced onion and cook for 3-4 minutes until softened. Add the minced garlic and cook for an additional 30 seconds until fragrant.
Transfer the cooked onions and garlic to the slow cooker. Place the seared chicken thighs on top.
Pour the chicken broth over the chicken and sprinkle dried thyme, dried rosemary, and smoked paprika evenly over the top.
Cover and cook on low for 5 to 6 hours, or until the chicken is tender and cooked through (internal temperature reaches 165°F).
About 30 minutes before serving, remove the chicken thighs from the slow cooker and cover to keep warm.
In a small bowl, whisk together the flour and heavy cream until smooth.
Stir the butter into the liquid in the slow cooker until melted. Gradually whisk in the cream and flour mixture. Cover and cook on high for 20-30 minutes, stirring occasionally, until the gravy thickens.
Return the chicken thighs to the slow cooker to warm through in the gravy for 5 minutes.
Serve the chicken topped with the rich gravy.