Put a large skillet on high heat, when hot, seal the chicken thighs until they are well browned. Remove from heat and reserve.
In the same pan, lower the heat, and add the onion and garlic in the oil that the chicken has released, if you see that it is not enough, add a little more. Let the onion caramelize slowly, about 10 minutes.
Add the tomato concentrate, take a couple of turns to mix well. Add chicken thighs, carrots and mushrooms.
Pour the red wine, the chicken stock, the bay leaves, the thyme and the rosemary. Season, cover and cook at low temperature for 2 hours. Take a look every half hour to see that it does not run out of broth and that the boil does not stop. If necessary add more hot chicken broth.
Once the time has passed, uncover it and turn up the heat, let it boil for 15-20 minutes to reduce the sauce. Remove the bay leaves, rosemary and thyme and discard.
Serve chicken and sauce over mashed potatoes.
If you wanted the sauce a little thicker, you could thicken it with a little cornstarch. In a bowl, add a teaspoon of cornstarch and a spoonful of the stew stock, disolve the cornstarch and then add to the stew. Take a couple of turns and let it boil until it thickens. If you wanted it very thick, add more cornstarch, but always keep in mind that when it gets a little cold it will thicken more.