Heat a large pot over medium heat, add oil. Once oil is hot, add carrots, celery, and onion to the pan and sauté, stirring occasionally, until translucent about 7-10 mins
While the vegetables are cooking, rinse and drain bag of lentils in a small-holed colander or in a bowl. Be sure to thoroughly look at lentils as you rinse. Look to remove any fine particles, dirt, stones that could be found in a bag of lentils.
Once vegetables are soft and translucent, add the garlic and red pepper flakes (optional) to the pot, stir for another 1-2 minutes until fragrant.
Add the lentils, broth, sweet potatoes and tomato sauce. Stir well, cover and bring to a low boil. Let simmer for 30 minutes. **
Add ketchup, and seasonings to the pot. Stir and continue to simmer for at least another 30 minutes.
**Note: You will definitely need to add more liquid as it simmers, as it can get too thick. I typically will alternate between adding water and broth**
After the 30 mins or less of simmering, taste the lentils. If they are still chewy, they need more time. Lentils and sweet potato should be tender. Add more seasonings to your liking. Again, you may have to add more liquid, use your judgment on the consistency.
Once the lentils are tender and the consistency is to your liking serve and enjoy!