In a large skillet, heat 1 tablespoon Vegetable oil over medium-high heat. Saute the mushrooms until they are browned. Remove from pan and set aside.
Add the remaining 1 tablespoon of oil to the pan and sauté the onion until translucent. Add carrots, celery, and bell pepper and cook over medium heat for 5 minutes.
Place the mushrooms back in the pan and add lentils, ketchup, Worcestershire sauce, and mustard. Turn heat to low and stir. Season to taste with salt and pepper.
Serve sloppy joe mixture between buns.
Notes
For more flavor, add 1/2 a vegetable bouillon cube to the pan with the onions.Recipe adapted from The Southern Vegetarian Cookbook