Heat the olive oil in a large skillet over medium-low heat. Add the sliced onions and cook, stirring occasionally, until soft and golden, about 20 to 25 minutes. Let them cool for a few minutes, then finely chop them.
Heat the oven to 375°F. Lightly grease the cups of a 12-cup muffin tin.
In a small bowl, stir together the breadcrumbs and milk. Let the mixture stand for a few minutes, until the breadcrumbs have absorbed the milk.
In a large bowl, combine the ground turkey, egg, soaked breadcrumbs, chopped caramelized onions, Worcestershire sauce, Dijon mustard, salt, black pepper, and 2 tablespoons of the ketchup. Mix gently just until combined.
Divide the mixture among 8 muffin cups, filling each about three-quarters full. Spread the remaining ketchup over the tops.
Bake until the meatloaf muffins are cooked through and reach 165°F in the center, about 20 to 25 minutes. Let them rest in the tin for 5 minutes before removing.