This is simply the best Slow Cooker Marinara! It is perfect for an easy meal at the end of a busy day. You can also freeze it to have it on hand when you need it.
Heat a skillet to medium high heat. Add avocado oil and allow to heat for a minute. Add onion and cook for 8 to 10 minutes. Drizzle with sweetener, Italian seasoning, red pepper flakes and salt. Reduce heat to low and cook for 10 more minutes. Add garlic and cook for one more minute.
Transfer to a slow cooker and stir in fire roasted tomatoes, chopped tomatoes, tomato paste and balsamic vinegar. Cook for 2 hours on high or 4 hours on low.