Preheat your oven to 180°C (350°F).
In a large bowl, combine ground chicken, grated onion, mayonnaise, breadcrumbs, milk, eggs, and seasonings.
Mix everything well until fully combined.
Lightly grease a loaf pan and transfer the mixture into it.
Smooth the top and spread ketchup evenly over the surface.
Bake for 50 minutes or until the loaf is cooked through and firm.
Let it rest for 10 minutes before slicing.
Serve warm with your favorite sides or sauces.