Heat oil in a medium Dutch oven over medium heat; add onion, bell peppers, and garlic. Cook and stir until vegetables have softened, about 5 minutes.
Mash 1/2 black beans in a small bowl; transfer to the Dutch oven with remaining whole black beans. Add diced tomatoes, 2 cups water, corn, lentils, quinoa, chili powder, brown sugar, paprika, salt, turmeric, oregano, cumin, red pepper flakes, and black pepper; stir to combine. Bring to a boil, then reduce heat to low, and simmer for 2 hours, stirring occasionally.
Whisk 3 tablespoons water and cornstarch together until completely dissolved, adding more water if necessary. Stir flour slurry into chili; simmer until thickened, about 15 minutes. Mash chili with a potato masher until reaches desired consistency.
Amy's Vegan Chili · Dinner Match Lab | Dinner Match Lab