1. In a large skillet, heat the olive oil over medium heat. Once hot, carefully add the rump roast. Sear on each side for 2 minutes. Remove the pan from the heat.
2. In a 6 quart slow cooker, lay the onion wedges on the bottom and add the tomatoes, carrots, beef broth, garlic, balsamic vinegar, thyme, rosemary, and salt. Add the seared beef and cook on low for 8-10 hours or on high for 6 hours.
3. To prepare the spaghetti squash, pierce the squash with a fork several times and score it lengthwise with a knife. Place in microwave and cook for 5 minutes. Remove the squash from the microwave with a towel or oven mitts. Place on a cutting board and carefully cut into the squash lengthwise. Scoop out the seeds and discard. Place each squash half into a glass baking dish, cut side down, and fill it with about an inch of water. Cook in microwave for 10 minutes. Carefully remove from microwave and place each squash half on a plate or cutting board lined with paper towels. Allow to cool for 15 minutes. Flip squash over and run a fork against the squash to make “spaghetti strands”. Place the prepared spaghetti squash in a serving bowl.
4. To serve, place the spaghetti squash on a plate or in a bowl and top with beef ragu. Top with grated Parmesan and chopped basil if desired. Store leftover beef ragu in an airtight container for up to 3 days in the refrigerator.