Coarsely chop 1 large yellow onion and 3 pickled peppers (cherry, Peppadew), flicking away any seeds.
In a large Dutch oven, heat ¼ cup extra-virgin olive oil over medium.
Add the onion and peppers, season with ¼ teaspoon salt, and cook until softened and starting to brown, 5 to 7 minutes.
Add 1 tablespoon chili powder and cook until fragrant, 1 minute.
Add 1 tablespoon pickled pepper brine and stir, scraping up browned bits, until nearly evaporated, 1 to 2 minutes.
Add 4 cups water, 1 cup red lentils, ½ cup ketchup, 2 tablespoons Dijon mustard, 1 tablespoon dark brown sugar, 1 tablespoon low-sodium soy sauce, and 1 teaspoon ground black pepper.
Season with 1 ½ teaspoons salt.
Bring to a simmer, then reduce the heat to medium-low and cook, stirring to unstick any lentils, until the lentils are tender and start to fall apart, 20 to 25 minutes.
If you want less slop, keep simmering until it’s as thick as you want.
If the mixture looks dry and the lentils aren’t done yet, add more water.
Eat the sloppy lentils—excuse me, Lennys—on toasted burger buns with more sliced pickled peppers.
