Heat olive oil in a skillet over medium heat and sauté the onions and garlic until soft, about 5 minutes.
Transfer the sautéed onions and garlic to the slow cooker. Add the crushed tomatoes, broth, Italian seasoning, and bay leaf, stirring to combine.
Cover and cook on low for 6-8 hours or on high for 3-4 hours.
In the last 30 minutes of cooking, add the tortellini and cover to cook until tender.
Stir in the heavy cream, then season with salt and pepper to taste.
Remove the bay leaf, garnish with fresh basil if desired, and serve hot.