In a large skillet, heat the olive oil over medium heat. Add the chopped yellow onion and green bell pepper. Sauté for about 5 minutes until they are softened. Add in the minced garlic and cook for an additional minute until fragrant.
Stir in the cooked red lentils and drained black beans into the skillet. Combine well, ensuring the beans and lentils are evenly distributed among the sautéed vegetables.
Pour in the canned tomato sauce, chili sauce, and maple syrup. Sprinkle in the chili powder, smoked paprika, and soy sauce. Stir everything together until well combined and heated through. Allow the mixture to simmer for about 10 minutes, stirring occasionally.
While the mixture simmers, lightly toast the whole wheat hamburger buns under the broiler or in a toaster for a couple of minutes until golden.
Once the sloppy joe mixture is ready, spoon generous portions onto the toasted buns. Top with finely chopped green onions for an added crunch and freshness. Serve immediately and enjoy the delightful flavors!