Preheat oven to 375°F. Coat an 8-inch loaf pan with cooking spray.
In the bowl of a food processor, pulse onion, carrots, celery and garlic until finely chopped, about 10 pulses.
In a medium bowl, combine oats, chile powder, thyme, salt and black pepper; set aside. In a large bowl, beat egg. Add sirloin to egg and mix thoroughly with your hands. Add oat mixture and onion mixture and mix with your hands until well combined. Spoon into loaf pan, smoothing top with the back of a spoon. Place pan on a large baking sheet and bake for 1 hour.
Prepare glaze: In a small bowl, combine remaining ingredients. Remove meatloaf from oven and pour half of glaze over top, distributing evenly; reserve remaining half. Return to oven and bake for 30 minutes, until an instant-read thermometer registers 160°F when inserted in center. Let cool for 10 minutes. Slice into 8 equal pieces and serve with remaining glaze for dipping.
This recipe is inspired by the traditional Cuban dish ropa vieja (Spanish for "old clothes"). Traditionally served with potatoes and chickpeas, you can also try it with rice, quinoa or corn tortillas. Using sweet onions keeps the flavor subtle.