Make the vegetable puree. Add the chopped sweet potatoes, carrots, cauliflower, and red bell pepper to a large pot and fill with water. Bring to a boil over medium heat and cook until the vegetables are fork tender, about 10-15 minutes. Once tender, drain the vegetables and transfer to a high-speed blender. Add the olive oil and milk and blend until smooth.
Put all ingredients in a slow cooker bowl. Stir to combine and cook on low for 8 hours. The color should go from orange to a deep red during the cooking process.
Serve or store. Enjoy immediately over pasta or allow to cool and store in mason jars in the refrigerator for up to 4 days or in the freezer for up to 3 months. This recipe makes 5 (24 ounce) jars of sauce.