In a large skillet over medium heat, melt the butter with the olive oil. Add the chicken to the skillet and cook until lightly browned, about 5 minutes per side.
Remove the chicken from the skillet and place in a 9x13 inch baking dish. Set aside.
Add the onion and mushrooms to the skillet and cook until the vegetables are tender, about 5 minutes.
Add the thyme and garlic and cook for one more minute.
Add the chicken broth and cream and bring to a boil. Reduce heat and simmer for 5 minutes.
Pour the sauce over the chicken in the baking dish.
Sprinkle with cheddar cheese.
Bake in preheated oven for 15 minutes, or until the chicken is cooked through and the cheese is melted and bubbly.
Garnish with fresh parsley and serve.
Interesting Facts
This dish is named after the famous Irish cook, Mary O'Neill.
The combination of mushrooms, onions, and cheese make this dish a comfort food classic.
This dish is easy to prepare, making it perfect for a family gathering or a weeknight meal.