1. Make Relish: In a medium bowl, combine tomatoes, parsley, thyme,1/4 teaspoon salt, 1/8 teaspoon pepper, 2 tablespoons olive oil, 1/4 cup onion and vinegar. Set aside.   2. In a separate bowl, combine mayonnaise, mustard and cayenne  pepper. Set aside.   3. Heat 1 tablespoon oil in a large nonstick skillet over medium heat. Add  mushrooms and remaining onions and  cook, stirring occasionally, about 7 minutes, until' onions are translucent. Stir in garlic and cook 1 minute more.    4. Place half of chicken in a food processor with a metal blade. Pulse a few times until chicken is ground. Add  egg, breadcrumbs, remaining salt and  pepper. Pulse twice to blend. Add remaining chicken and pulse just until chicken is cut into small cubes but not  ground. Turn mixture out into a bowl and stir in mushroom-onion mixture. With clean, wet hands, shape into 4 patties. Set aside.   5. In same skillet, heat remaining oil over medium heat and cook chicken burgers about 5 minutes per side, until  cooked through. Spread mayonnaise  mixture on rolls and serve burgers  with tomato relish.    Makes 4 servings. Per serving: 600 calories, 30.5 9 total fat. 4.5 g saturated fat, 127 mg cholesterol,   1,222 mg sodium, 44 g carbohydrates, 37 9 protein, 110 mg calcium, 3 9 fiber 0