In large saucepan, heat oil and butter over medium. Add onion, carrot, celery, and salt; cook until onion has softened, about 5 minutes. Add garlic and heat for 1 minute. Stir in tomato paste, dried thyme, red pepper flakes, and black pepper; heat for 1 minute. Pour in wine, bring to boil, and simmer for 3 minutes. Place canned tomatoes and 1 cup water into pan. Raise heat to bring to boil; add drained mussels, cover, and cook over medium heat for 3 minutes, or until shells have popped open. Set aside.