Rinse the chicken breasts under cold water and pat dry with paper towels.
Cut the breasts into 4 equal pieces. Season both sides with salt and pepper.
Heat 2 tbsp olive oil in a pan. Fry the chicken pieces over medium heat for 5–7 minutes on each side until golden brown and cooked through (internal temperature should reach 75°C). Remove chicken and keep warm.
In the same pan, add 1 more tbsp olive oil. Sauté finely chopped onion until translucent.
Add minced garlic and cook for another minute until fragrant.
Add thinly sliced mushrooms. Cook on high heat for about 5–7 minutes until moisture evaporates and mushrooms turn golden.
Sprinkle flour over the mushrooms and stir well. Cook for 1 minute, stirring constantly to avoid burning.
Gradually pour in chicken broth while stirring to avoid lumps. Simmer for 2–3 minutes until sauce thickens slightly.
Reduce heat and add cream. Cook gently for 5 minutes until the sauce is creamy and thick. Season with salt and pepper to taste.
Meanwhile, peel potatoes, cut into medium cubes, and boil in salted water for 15–20 minutes until soft.
Drain the potatoes and return to the pot. Add butter and let it melt. Pour in warmed milk and mash until smooth. Season with salt and pepper.
To serve, place a portion of mashed potatoes on a plate, top with the chicken breast, and spoon the brown mushroom sauce over it. Garnish with chopped parsley.