Preheat your oven to 350°F (180°C).
Trim the chicken thighs by removing the skin and excess fat. Season the thighs generously with salt and pepper.
Heat a large frying pan over medium-high heat. Add olive oil and allow it to heat up. Place the seasoned chicken thighs in the pan.
Brown the chicken thighs for about 10 minutes.
Remove the chicken from the frying pan and transfer them to a glass casserole dish.
Pour out most of the fat from the pan. Add the red onion and cook for about 2 minutes until it softens.
Add the sliced mushrooms and cook for about 5 minutes.
Stir in the ground rosemary.
Spread the cooked mushrooms and onions over the chicken thighs in the casserole dish.
Roast in the preheated oven for 35 to 45 minutes.
Serve your Chicken Thighs with Mushrooms hot, spooning any juices from the dish over the top.