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.
Place the sliced onion and minced garlic in the bottom of the slow cooker.
Arrange the seared chicken thighs on top of the onions and garlic in the slow cooker.
Pour the chicken broth over the chicken and sprinkle dried thyme and dried rosemary evenly.
Cover and cook on low for 6 hours, until the chicken is tender and cooked through.
When the chicken is done, carefully remove the chicken thighs from the slow cooker and set aside, keeping warm.
Pour the cooking liquid from the slow cooker through a fine mesh strainer into a medium saucepan, pressing on the solids to extract as much liquid as possible.
In the saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes until lightly golden, stirring constantly to make a roux.
Gradually whisk in the strained cooking liquid and bring to a simmer. Cook, whisking frequently, until the gravy thickens, about 3-5 minutes.
Stir in the heavy cream and cook for an additional 2 minutes until the gravy is smooth and creamy.
Return the chicken thighs to the saucepan to coat with the gravy, or serve the gravy spooned over the chicken on plates.
Garnish with chopped fresh parsley before serving.