Preheat oven to 300°. Season beef all over with salt and pepper. In a large ovenproof pot over medium-high heat, heat oil until shimmering. Working in batches, arrange beef in a single layer and cook, turning occasionally, until deeply browned on each side, 8 to 10 minutes. Transfer to a plate.

Reduce heat to medium and pour more oil into pot if it seems dry. Add onion, bell pepper, jalapeño, and garlic; season with salt. Cook, stirring occasionally, until vegetables soften and begin to caramelize, about 10 minutes. Add chili powder, cumin, and paprika and cook, stirring occasionally, until fragrant, about 1 minute. Stir in tomatoes, chiles, and brown sugar, scraping up any browned bits from bottom of pot (this is where the extra flavor hides); season with salt and pepper. Cook, stirring, until thickened slightly, about 3 minutes.

Add broth and bring to a boil. Reduce heat to medium-low and return beef and any accumulated juices to pot. Cover pot, leaving lid slightly ajar.

Transfer to oven and bake chili until meat is very tender, 2 to 2 ½ hours.

Meanwhile, in a small bowl, mix masa harina with ¼ cup water to form a paste.

Return chili to stovetop. Stir in masa harina paste and cook over low heat, stirring occasionally, until thickened, about 5 minutes; season with salt and pepper. Add more chiles, if desired, for a spicier stew.

Divide chili among bowls. Top with cheddar and scallions.