Heat oil in a large frypan over medium heat. Add the chicken thighs, crushed garlic and Italian mixed herbs. Brown the chicken thighs on both sides then add the tomatoes and portobello mushrooms. Season with salt and pepper. Reduce heat to low, cover and simmer for about 30 minutes.
Strain off about half of the liquid from the frypan and pour in the Burgundy wine. Cover and simmer for another 30 minutes. Remove the chicken, tomatoes and mushrooms to a square baking dish. Cover with sliced onion and set aside.
Preheat the oven to 180 degrees C. Bring the liquid remaining in the frypan to the boiling point over medium-low heat. Gently whisk in the cream and Gruyere cheese.
Remove from the heat and stir until smooth. If the sauce is not thick enough whisk in the cornflour then simmer over very low heat until it thickens. Pour the sauce over the chicken in the baking dish.
Bake for 30 minutes in the preheated oven until onions are tender. Let rest for 5 minutes before serving.