Peel the onions and garlic and chop into fine pieces. Heat 1 tablespoon of vegetable oil in a pan. Sauté the onions and garlic over medium heat for 2–3 minutes until transparent. Cut the sundried tomatoes into small pieces and stir in together with 1 sprig each of thyme and oregano. Spread the onion-tomato mixture in an oven-proof dish evenly.
Wash and dry chicken breast fillets and rub with vegetable oil on all sides. Season with salt and pepper and sear briefly on all sides in a coated pan. Place the fillets on the onion-tomato layer in the baking dish.
Preheat oven to 170°C (approx. 340°F) . Mix mustard, honey and walnut oil and pour over the fillets. Wash and cut the tomatoes in halves. Add to the chicken in the dish together with the remaining herbs, season with salt and pepper and cover with aluminium foil. Let roast in the oven for about 15 minutes. Remove the foil after about 7 minutes and serve the chicken hot.