Start by dicing all your vegetables: mushrooms, onion, carrot, celery, and green bell pepper. Having everything prepped and ready to go makes the cooking process a breeze.
In a large skillet, heat 1 tablespoon of vegetable oil over medium heat. Add the diced onions, carrots, celery, and green bell pepper. Sauté for about 5 minutes, or until the vegetables are tender. Then, add the diced mushrooms and cook for an additional 3-4 minutes until they release their moisture and start to brown.
Stir in the cooked lentils, ketchup, vegetarian Worcestershire sauce, and mustard. Mix well, ensuring that everything is evenly coated. Let the mixture simmer for about 5-7 minutes, allowing the flavors to meld together. Season with sea salt and black pepper to taste.
While the filling is simmering, you can lightly toast the whole wheat buns. This adds a nice crunch and prevents them from becoming too soggy.
Spoon the hearty lentil mixture onto the toasted buns, and serve immediately. For an extra touch, consider topping your Vegetarian Sloppy Joes with sliced pickles or a sprinkle of fresh herbs.
