Preheat oven to 350°F. Line a rimmed baking sheet with foil and grease it.
Make the glaze: In a bowl, whisk ketchup, brown sugar, vinegar, and ½ tsp mustard. Set aside.
Heat oil in a skillet. Cook onion until soft (5 mins). Add garlic and cook 1 min more. Let cool.
In a large bowl, whisk salt, pepper, paprika, thyme, Worcestershire, 1½ tbsp mustard, eggs, and ¼ cup of the glaze. Stir in onion mixture.
Add ground turkey and breadcrumbs. Mix with hands until just combined.
Shape into two loaves on the baking sheet. Spread remaining glaze over the tops.
Bake for 45 minutes, or until internal temperature reaches 165°F. Rest for 10 minutes before slicing.