Preheat oven to 425 degrees. Destem peppers and slice in half, poke each tomatillo with a knife and quarter the onion reserving half for the lentil filling. Leave garlic cloves shelled. Spread all ingredients out on a baking sheet lined with parchment paper and roast for 45 minutes or until browning occurs.
While the veggies roast, rinse the lentils. Add vegetable broth to a medium-sized pot and bring to a boil. Add rinsed lentils, cover, and simmer for about 25 minutes until the lentils are tender. Set aside.
Meanwhile, chop the reserved half of the onion and add to a skillet with olive oil. Sauté for five minutes or until onions are translucent and set aside.
Once the salsa ingredients are cool enough to handle, reserve one half of a pasilla pepper and add the rest of the ingredients to a food processor, pulsing until you reach a salsa-like consistency.
Chop the reserved pasilla pepper. Bring lentils back to the burner and combine salsa, reserved pepper, and sautéd onion. Add Mexican oregano and salt and pepper to taste. Simmer until the mixture resembles a sloppy joe consistency (about 10 minutes). Remove from heat and add optional lime juice or fresh cilantro
Toast the buns and add a healthy scoop of lentil sloppy joe mix to the middle.