Begin by heating the olive oil in a skillet over medium heat. Add the diced onion and sauté for about 3-4 minutes until it becomes translucent.
Add the minced garlic, diced carrots, and diced celery to the skillet. Sauté for an additional 3-4 minutes until the vegetables start to soften.
Transfer the sautéed vegetables to the slow cooker. Add the crushed tomatoes, vegetable broth, dried basil, dried oregano, salt, and pepper. Stir to combine.
Cover the slow cooker and cook on low for 6 hours or high for 3 hours.
About 20 minutes before serving, add the tortellini to the slow cooker. If using fresh tortellini, cook for 3-4 minutes; if using frozen, cook for about 5-6 minutes.
Stir in the fresh spinach and heavy cream (if using) just a few minutes before serving, allowing the spinach to wilt.
Taste and adjust seasoning if necessary.
Serve hot, garnished with additional basil or grated Parmesan cheese, if desired.