Preheat the oven to 375°F. Line a rimmed baking sheet with parchment paper or a silicone baking mat.
Cut the peppers in half vertically and remove the ribs and seeds. Place the peppers on the prepared baking sheet and bake for 20 to 25 minutes, until they are tender.
While the peppers are in the oven, make the sloppy Joe mixture: Heat the olive oil in a large skillet over medium heat. When the pan is hot, add the onion, celery, and garlic. Cook until the onion and celery are soft and the garlic is fragrant.
Add the ground beef, breaking it up with a heatproof spatula, and cook until browned, about 8 minutes.
Mix in the tomato sauce, ketchup, Worcestershire sauce, Dijon mustard, chili powder, cumin, and salt. Simmer for 5 to 7 minutes, until the sauce has thickened.
Stuff the sloppy Joe mixture into the pepper halves and garnish with the parsley.