Oven: Preheat oven to 350°F (175°C).
Sauté the aromatics: In a skillet over medium heat, sauté the diced onion (1/2 cup) in a drizzle of olive oil until soft (about 4–5 minutes). Add the minced garlic (2 cloves) and cook for another 30 seconds. Let cool slightly before adding to the meat mixture.
Mix the meatloaf: In a large bowl, combine the ground beef (2 pounds), sautéed onion and garlic, onion soup mix (1 envelope), breadcrumbs (3/4 cup), eggs (2), milk (2/3 cup), tomato paste (2 tablespoons), Dijon mustard (1 tablespoon), ketchup/BBQ sauce (1/2 cup), Worcestershire sauce (2 tablespoons), smoked paprika (1 teaspoon), black pepper (1/2 teaspoon), and red pepper flakes (1/4 teaspoon). Mix just until everything is combined—don’t overwork it.
Shape and place: Line a rimmed baking sheet or 9x13-inch dish with parchment paper. Transfer the mixture and shape into a loaf, roughly 8x4 inches.
Bake (Part 1): Bake uncovered for 40 minutes.
Prepare the sauce: In a small bowl, mix together ketchup (1/2 cup), brown sugar (2 tablespoons), Dijon (1 tablespoon), vinegar (1 teaspoon), and cayenne (pinch) (if using).
Glaze and finish baking: Spoon the sauce over the meatloaf and bake for an additional 20 minutes, or until the internal temperature reaches 160°F (71°C).
Rest and serve: Let the meatloaf rest for 5–10 minutes. Slice and garnish with fresh parsley before serving.