Heat olive oil in a large skillet or pot over medium heat. Add onion and bell pepper. Cook 5–6 minutes until softened and lightly browned. Add garlic and cook 30 seconds until fragrant.
Stir in tomato paste and cook 1 minute to caramelize. Add chili powder, smoked paprika, and cumin; cook another 30 seconds to bloom the spices.
Stir in lentils, crushed tomatoes, soy sauce, and vegetable broth. Bring to a simmer.
Reduce heat to medium-low, cover slightly ajar, and cook 25–30 minutes, stirring occasionally. Add splashes of broth if it gets too thick. Lentils should be tender but not falling apart.
Stir in maple syrup, mustard, and apple cider vinegar. Taste and adjust salt, pepper, sweetness, and tang. You're aiming for bold, slightly sweet, and pleasantly smoky.
Let it bubble uncovered 3–5 minutes if you need a thicker, scoopable consistency. This is your sloppy sweet spot.
Lightly toast whole-wheat buns (skillet or broiler) for sturdiness and flavor. A little crisp prevents sogginess.
Spoon generous portions onto buns. Add pickles or coleslaw if you're extra. Eat immediately and embrace the mess—napkins are not optional.