Preheat oven to 350 degrees F. In a large skillet, saute onions and garlic in olive oil. When onions are soft, add diced tomatoes, herbs, salt and pepper. Reduce heat and simmer for 15 minutes. Bring a large pot of water to a boil. Add 1/2 of potatoes and boil for 5 minutes. Remove potatoes from water and add remaining potatoes. Butter a 2 quart cassarole and layer with 1/2 of the potatoes. Top with 1/2 of tomato mixture. Sprinkle with 1/2 cup Parmesan cheese. Repeat with remaining potatoes and tomato mixture. Combine broth and cream in a medium saucepan over medium heat. Add flour, cook and stir until mixture starts to thicken. Pour broth mixture over potato mixture. Sprinkle with remaining Parmesan cheese. Cover and bake for 45 minutes. Remove cover and bake 30 minutes longer. Serve.