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.
Cook until internal temperature registers 155˚F on quick-read thermometer, about 1 hour 15 minutes.
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 and internal temperature registers 165°F on quick-read thermometer, about 10 minutes more. Remove from oven; let rest 10-15 minutes.
To serve, slice meatloaf into 6 portions; serve warm.
For individual meatloaves that cook quickly, form meat mixture into six 4-oz. meatball shapes. Place meatloaf balls into greased muffin tins, cover with foil; cook until internal temperature registers 165˚F on quick read thermometer, about 45 min. Then follow step 4 (above). Serve by placing each mini meat loaf on plate.