Preheat the oven to 180°C, fan 160°C, gas 4. Put the oil in a large, deep baking tray with the tomatoes, red onions, garlic cloves, paprika seasoning, yellow pepper, red wine vinegar and stock. Season with freshly ground black pepper and mix well. Sit the chicken thighs on top and bake for 1 hour.
About 10 minutes before the end of cooking time, cook the mashed potato in the microwave to pack instructions.
Squeeze the soft garlic cloves out from their skins and into the pan, then divide the chicken, vegetables and sauce between 4 plates and serve with the mash.