Heat oil in a large Dutch oven over medium-high. Add onion and bell pepper and cook, stirring occasionally, until slightly softened, about 6 minutes. Add garlic, chili powder, cumin, and 1 tsp. salt, stirring constantly for 1 minute. Add stock, tomatoes, and lentils and bring to a boil over high. Reduce heat to medium and simmer, partially covered, until lentils are tender, about 25 to 30 minutes. Add kidney beans, brown sugar, sherry, and remaining 1/2 teaspoon salt. Reduce heat to low and stir occasionally until beans are heated through, about 5 minutes. Spoon chili into bowls and top with cheese, sour cream, cilantro, and jalapeno slices.