Toss sirloin, and 2 tablespoons flour in bowl. Melt 2 tablespoons butter in large Dutch oven over medium-high heat. Cook until beef is browned; transfer with slotted spoon to bowl.
Add celery, carrot, onion, garlic, thyme and white pepper to Dutch oven. Cook, stirring, 3 minutes, until vegetables are lightly browned.
Add broth, tomatoes, 1 cup water, reserved beef, and 1/2 teaspoon salt. Bring to boil; reduce heat to medium and cook 20 minutes, until vegetables are tender. Add beef ravioli; cook 10 minutes, stirring occasionally.
Meanwhile, combine 2 tablespoons flour and 1 tablespoon cornstarch with 2 tablespoons water in cup until smooth. Stir into soup; bring to boil and boil 1 minute.