Heat olive oil in a large pot over medium heat. Add diced onion, bell pepper, and minced garlic. Sauté until vegetables are soft, about 3–4 minutes.
Add tomato paste and stir continuously for 1 minute until fragrant.
Add lentils and water or broth. Bring mixture to a boil, then reduce heat and simmer for 10–15 minutes until lentils are tender.
Stir in mustard, maple syrup (or brown sugar), apple cider vinegar, chili powder, and salt. Simmer for an additional 3–5 minutes to allow flavors to meld.
Serve warm on burger buns or toasted bread.