Preheat oven to 375°F. Grease a 9x5-inch loaf pan or line it with parchment.
Warm olive oil in a skillet over medium heat. Soften onion for 3–4 minutes; let cool.
Soak breadcrumbs in milk in a large bowl for 2–3 minutes until moistened.
Whisk egg with ketchup, Worcestershire, Dijon, parsley, garlic, Parmesan, salt, pepper, and Italian seasoning.
Add ground chicken and cooled onion to the bowl. Mix gently until just combined; do not overwork.
Press mixture into the prepared pan and smooth the top.
Stir glaze by combining ketchup, brown sugar, and vinegar. Spread half over the loaf.
Bake 30 minutes. Brush with remaining glaze and bake 12–18 minutes more, until internal temp hits 165°F.
Rest 10 minutes in the pan. Slice and serve warm.