Heat oven to 400˚F. In small bowl, mix together tomato sauce, brown sugar, 1 tbsp. adobo and mustard; set aside.
In large bowl, mix together meatloaf mix, ¾ cup tomato sauce mixture, breadcrumbs, onions, beaten egg, garlic and remaining adobo until well combined. Form meat mixture into oblong loaf shape. Transfer meat to greased 2-lb. loaf pan; cover with foil.
Transfer loaf pan to oven; cook until internal temperature registers 165˚F on quick-read thermometer, about 1 hr., 30 min.
Remove pan from oven; discard foil. Carefully drain and discard fat in pan. Heat broiler. Spoon remaining tomato sauce mixture on loaf. Cook until sauce turns dark golden brown, about 10 minutes more. Remove from oven; let rest 10-15 min.
To serve, slice meatloaf into 6 portions; serve warm.