Heat the oil in a large pot over medium heat. Once hot, add the onion, fennel (or celery), and carrot. Sauté gently for 8 minutes, stirring occasionally, until the vegetables are soft. Stir in the salt and pepper — don’t be tempted to turn the heat up to speed this up or the veggies won’t get as sweet.
Add in the cabbage and kale (if using), and sauté for another 5 minutes until wilted. Then stir in the garlic, fennel seeds, smoked paprika, and red pepper flakes. Cook for 1 more minute until the fennel seeds are fragrant.
Stir in the vegetable broth, diced potatoes, crushed tomatoes, and lentils. Bring to a simmer, cover (with the lid slightly ajar), and simmer on medium-low for 30-40 minutes, or until the lentils are tender but not mushy.
Taste and adjust as needed, adding more salt for overall flavor or red pepper flakes for heat.