Add tomatoes, celery, carrots, onions, oregano, basil, chicken broth and bay leaf to your slow cooker.
Stir until combined, cover and cook on low for 6-8 hours or until vegetables are tender.
Thirty minutes before serving, melt butter over low heat in a medium saucepan.
Whisk flour into the melted butter to create a roux. Whisk mixture for about 2-3 minutes, or until golden brown. Whisk in one cup of soup from your slow cooker until smooth. Whisk in an additional cup of soup from the slow cooker. When smooth, add mixture back to the slow cooker.
Add Parmesan cheese, whipping cream and season with salt and pepper.
Let cook for 30 more minutes. Remove bay leaf before serving.