Heat the smoker to 250°F. In a large bowl, mix the ground beef, egg, breadcrumbs, milk, Worcestershire sauce, salt, black pepper, and smoked paprika just until combined. Shape the mixture into a compact loaf and place it on a foil-lined tray.

Make the glaze by stirring together the bourbon, ketchup, brown sugar, and Dijon mustard in a small bowl. Set aside half for serving, keeping it separate from the glaze used on the raw meatloaf.

Place the loaf in the smoker and cook until the center reaches 145°F, about 1 hour. Brush the top and sides with the remaining glaze, then continue smoking until the center reaches 160°F. Rest the meatloaf for 10 minutes before slicing.

While the meatloaf smokes, heat half of the olive oil and the butter in a large skillet over medium-low heat. Add the onions and cook, stirring occasionally, until soft and deeply golden, about 30 to 40 minutes. If they begin to stick, stir in a small splash of the listed beef broth.

Stir the flour into the onions and cook for 1 minute. Gradually whisk in the remaining beef broth. Simmer, stirring, until the gravy thickens. Keep it warm over low heat.

Toss the carrots, potatoes, and parsnip with the remaining olive oil. Spread them on a baking sheet and roast at 425°F, turning once, until browned and tender, about 30 to 40 minutes. If your smoker is set up to accommodate a roasting pan at that temperature, you can roast them there instead.

Serve slices of meatloaf with the caramelized onion gravy and roasted root vegetables. Offer the reserved bourbon glaze at the table.