In large skillet, heat oil over medium heat; cook onions, celery, carrot, red pepper, garlic, chili powder, salt and pepper, stirring occasionally, for 5 to 8 minutes or until vegetables have softened. Add tomato paste; cook, stirring, for 1 minute. Stir in diced tomatoes; bring to simmer. Cook for 6 to 8 minutes or until sauce thickens slightly and vegetables are tender; stir in chicken, cook until heated through. Spoon mixture into toasted buns with Swiss cheese. Serve with vegetable sticks and blue cheese dip.