Heat the olive oil in a large skillet over medium heat. Season the chicken thighs with salt, pepper, smoked paprika, and cayenne (if using). Once the oil is hot, add the chicken thighs, skin-side down, and sear for about 5-6 minutes until golden brown. Flip and cook for an additional 5 minutes. Remove the chicken and set aside.
In the same skillet, add the diced onion, bell pepper, and garlic. Sauté for about 3-4 minutes until the vegetables are softened.
Stir in the mushrooms and cook for another 2-3 minutes. Sprinkle the flour over the vegetables and stir to combine, cooking for 1 minute.
Gradually add the chicken broth, stirring continuously to avoid lumps. Bring the mixture to a simmer and allow it to thicken for about 2-3 minutes.
Return the chicken thighs to the skillet, nestled into the gravy. Cover the skillet, reduce the heat to low, and simmer for 25-30 minutes until the chicken is cooked through and tender.
While the chicken is cooking, prepare the rice. In a medium saucepan, bring 2 cups of water to a boil. Add the rice and a pinch of salt. Stir, cover, and reduce to a low simmer. Cook for about 18-20 minutes or until the rice is fluffy and water is absorbed.
Once the chicken is done, serve it hot over the cooked rice. Spoon the savory gravy over the chicken and rice, and garnish with fresh parsley if desired.