Heat oil in large Dutch oven over medium-high heat. Sprinkle chicken thighs evenly with 1 teaspoon salt and 1/2 teaspoon pepper.
Add butter to the drippings and melt. Add onion and cook, stirring often, until translucent, about 3 minutes. Add garlic; cook for 1 minute. Add rice; cook, stirring to coat grains in drippings and lightly toast, about 1 minute.
Stir in 3 cups stock and remaining salt and pepper. Bring to a boil. Use a wooden spoon to scrape any browned bits from the bottom of the Dutch oven. Place the chicken thighs over the rice.
Bake in the preheated oven, uncovered, until all liquid has been absorbed and chicken is no longer pink at the bone and the juices run clear, 45 to 55 minutes. Add more stock, 1/4 cup at a time, if liquid is absorbed before rice and chicken are tender. Sprinkle with parsley and squeeze lemon juice over the top.
