Heat the oven: Preheat the oven to 375°F and line a rimmed baking sheet with foil. A free-form loaf on a sheet pan browns on all sides, while a loaf pan traps steam and leaves the outside pale and soft.

Cook the vegetables: Heat the olive oil in a skillet over medium heat. Add the onion and cook 5 minutes until soft, then add the mushrooms and cook 8 to 10 minutes more, stirring occasionally, until they release their liquid and the pan goes dry again. Stir in the garlic, cook 1 minute, then set aside to cool for 5 minutes.

Soak the panko: In a large bowl, stir the panko and milk together and let it sit 5 minutes, until the crumbs swell and the mixture looks like a loose paste. This is the step that keeps the meatloaf moist, so don't rush past it.

Mix the loaf: Add the beaten eggs, Worcestershire sauce, salt, pepper, thyme, and the cooled mushroom mixture to the bowl and stir until combined. Add the ground turkey and mix with your hands just until you no longer see streaks of plain meat. Overmixing packs it tight and makes it dense.

Shape it: Turn the mixture onto the lined sheet pan and shape it into a loaf about 9 inches long and 5 inches wide. Keep the top gently rounded rather than flat so the glaze pools evenly.

Glaze and bake: Whisk the ketchup, brown sugar, vinegar, and Dijon together. Brush half of it over the loaf and bake 35 minutes. Brush on the remaining glaze and return it to the oven for 15 to 20 minutes more.

Check the temperature: The turkey meatloaf is done when an instant-read thermometer inserted into the center reads 165°F. Ground turkey is poultry, so 165°F is the safe temperature, not the 160°F used for ground beef.

Rest before slicing: Let the loaf rest on the pan for 10 minutes before cutting. Slicing it hot lets the juices run out onto the pan instead of staying in the meat. Sprinkle with parsley and slice thick.