Finely dice the yellow onion and mince the garlic. Sauté the onion in a skillet over medium heat until translucent and soft, about 5-7 minutes. Set aside to cool.

In a large bowl, combine breadcrumbs with milk and let sit for 5 minutes. Add ground beef, cooled onion, minced garlic, eggs, Worcestershire sauce, horseradish mustard, and parsley. Mix gently to avoid overmixing.

Transfer mixture onto a lined baking sheet or loaf pan. Shape into a loaf with an indentation on top to prevent doming.

In a saucepan, combine ketchup, brown sugar, honey, molasses, vinegar, and mustard. Heat over medium-low, stirring until thickened, about 10-15 minutes.

Preheat smoker or grill to 250°F (120°C). Ensure enough wood chips or pellets for flavor.

Place the meatloaf on the smoker rack. Smoke for about 1 hour without glazing.

After 1 hour, apply the glaze generously and continue to smoke for 1.5 to 2 hours, basting every 30 minutes.

Check the internal temperature with a thermometer; it should read 160°F (70°C). Let it rest for 10-15 minutes before slicing.