Preheat your oven to 400 degrees Fahrenheit. Lightly grease a standard 12-cup muffin tin.
In a large bowl, combine the breadcrumbs and milk. Let stand for 5 minutes to soften the breadcrumbs.
Add the ground beef, beaten egg, chopped onion, minced garlic, salt, pepper, 1/4 cup ketchup, Worcestershire sauce, and 1 tablespoon Dijon mustard to the breadcrumb mixture.
Mix gently with your hands until just combined. Do not overmix the meat.
Divide the meat mixture evenly among the 12 muffin cups, pressing lightly to form a compact shape.
In a small bowl, whisk together the glaze ingredients: 1/4 cup ketchup, maple syrup, 1 tablespoon Dijon mustard, and apple cider vinegar.
Brush half of the glaze mixture evenly over the tops of the meatloaves.
If using the optional side vegetables, toss the chopped root vegetables with olive oil, salt, and pepper. Arrange these vegetables around the edges of the muffin tin, or spread them on a separate small baking sheet.
Bake for 15 minutes.
Remove the tin from the oven and brush the remaining glaze over the tops.
Return to the oven and bake for another 5 to 10 minutes, or until the internal temperature reaches 160 degrees Fahrenheit. Total baking time is about 20 to 25 minutes.
Let the mini meatloaves rest in the tin for 5 minutes before carefully removing them. Serve immediately with the roasted vegetables.
Notes
You can prepare the meat mixture and place it in the muffin tin up to 24 hours ahead. Cover and refrigerate. Add 5 minutes to the initial baking time if cooking straight from the refrigerator.
These individual meatloaf servings freeze well. Cool completely, then place them in an airtight container separated by parchment paper. Reheat in the oven or microwave.
For a bacon-wrapped version, wrap each portion with half a slice of bacon before baking.