Heat the oil in a large pot over medium heat.
Add in the diced onions, carrots, and celery, and saute for about 8-10 minutes until onion is soft and translucent.
Add the minced garlic and saute for a couple of minutes. Keep an eye on the garlic and stir as it will brown quickly (1-2 minutes is enough to cook up the garlic).
Add in the lentils and stir, combining it with the soffritto mixture (about 2-3 minutes).
Add in the tomatoes and use a wooden spoon to break them up and combine them with the lentils (if using canned plum tomatoes, break them up before adding, and remove the tough white core). Let this simmer together for a few minutes and season with salt (and pepper, if you’re using). If you’re not using a low sodium broth, wait to add salt until after you add your broth.
Next, add in the broth and bay leaf.
Bring to a boil for a few minutes and then lower the heat to a simmer.
Check the seasoning and add more salt (if needed)and pepper and let it simmer for about 30 minutes.
Stir in kale during last 10 minutes of cooking. You just want the kale to be wilted, but cook it more if you don’t mind it on softer side.
If soup is thickening too much, add a bit of water (start out with 1/4 cup and check how consistency is). The lentils will thicken up as they cook.
When ready to serve, remove the bay leaf.
Ladle into serving bowls and drizzle on some olive oil.
Add in red pepper flakes, if using.
SLOW COOKER DIRECTIONS:
Add the first 12 ingredients (through the salt + pepper) to a large 6-quart slow cooker, and stir to combine.
Cook for 4-5 hours on high or 8-10 hours on low, until the lentils are tender and cooked through.
Stir in the kale during the last hour of cooking.
Taste, and season with additional salt and pepper if needed. Remove the bay leaves.
Serve warm, garnished with optional toppings if desired.