Creamy Herbed Chicken Salad with Tomatoes

Rinse the tomatoes and cut into quarters. Rinse the chicken breasts, pat dry and cut into cubes. Heat 1 tablespoon butter in a skillet, season the chicken with salt and pepper and saute chicken until cooked through. Remove from the pan and set aside. Peel and finely chop the onion and garlic. Rinse the celery, remove the strings, halve lengthwise and thinly slice crosswise. Heat the remaining butter in a skillet and saute the onion, garlic and celery until the onion is translucent. Add the broth and cook until the liquid has evaporated, 3-5 minutes. Remove from the heat, transfer to a bowl and cool to room temperature. Add the basil, balsamic vinegar, mayonnaise and sour cream and mix well. Add the chicken and toss to coat.