Preheat the oven to 400°F (204°C). Lightly oil a rimmed baking sheet or a 9-inch by 13-inch baking pan lined with aluminum foil.

Heat the oil in a large skillet over medium-low heat. Add the chopped onion and cook, stirring occasionally, until softened, about 5 minutes.

Add the minced garlic and cook for about 1 minute until fragrant.

Stir in the chopped mushrooms, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Cook for about 10 minutes, until the mushrooms release their liquid and it cooks away.

Transfer the onion and mushroom mixture to a large bowl. Stir in the Worcestershire sauce and 3 tablespoons of the ketchup. Let cool for 5 minutes.

In a small bowl, combine the breadcrumbs and milk.

Add the breadcrumb mixture and beaten eggs to the mushroom mixture and stir gently to combine.

Add the ground turkey, remaining 1/4 teaspoon salt, and remaining 1/4 teaspoon black pepper. Mix gently until just combined; the mixture will be very wet.

Form the mixture into a 9-inch by 5-inch oval loaf in the center of the prepared baking sheet.

Spread the remaining 4 tablespoons of ketchup evenly over the top.

Bake for about 50 minutes, until an instant-read thermometer inserted into the center reads 170°F.

Let the meatloaf rest for 10 minutes before slicing and serving.