1tablespoonneutral vegetable oil, such as safflower or grapeseed*
1large white or yellow onion, chopped
2stalks celery, chopped
1 28-ouncecan fire-roasted, diced tomatoes
4cupswater or low-sodium vegetable broth
1lbbrown or green lentils, picked over and rinsed
3tablespoonschipotle peppers in adobo sauce, finely chopped (use 2 tablespoons if you're sensitive to heat)
3/4teaspoonsalt, more as needed
1tablespoonmaple syrup, agave syrup, or brown sugar
1tablespoonapple cider vinegar
For most depth of flavor, begin by heating the oil in a roomy skillet over medium heat. Add the onion and celery. Sauté, stirring occasionally, for 5-7 minutes, or until the onions are gently browning. Add the garlic and sauteé for one minute more. Add this mixture to the slow cooker (or a multi-cooker with a slow cooking function), then add all of the remaining ingredients.
Alternately, add everything but the oil to the slow cooker. Cook on high for 4 hours or low for 7-8 hours. If the lentils are too thick, add extra water to thin them to your liking (I like them to be thick, rather than soupy). Taste and adjust vinegar and salt to taste. Store in an airtight container in the fridge for up to 5 days, or freeze for up to 6 weeks.