Heat 2 tablespoons of olive oil in a large non-stick skillet over medium-high heat. Once the oil shimmers, add 1 diced medium yellow onion and 1 chopped green pepper. Sauté for about 8 minutes until the vegetables are softened and slightly translucent. Stir in 2 minced garlic cloves and continue sautéing for another 2 minutes until fragrant.

Next, stir in 2 cups of cooked lentils, 1 tablespoon of chili powder, 1 teaspoon of oregano, and a pinch of cayenne pepper to the skillet. Pour in 1 can of crushed tomatoes along with 2 tablespoons of tomato paste, 1 tablespoon of maple syrup, and 1 tablespoon of yellow mustard. Mix everything well and bring the mixture to a gentle simmer. Cover and let cook for about 10 minutes.

While the lentil filling is simmering, preheat your oven to 350°F (175°C). Cut kaiser rolls or sesame buns in half and place them on a baking sheet. Toast the buns in the oven for about 5 minutes or until they are lightly golden and crisp.

Once the lentil filling is heated through and thickened, turn off the heat. Using a spoon, generously scoop the savory lentil sloppy joe mixture onto the toasted buns. Serve immediately.