In a large bowl, stir together the breadcrumbs and milk. Let them stand for 2 minutes so the breadcrumbs absorb the milk.
Add the ground turkey, egg, grated onion, parsley, thyme, salt, and black pepper. Mix gently with your hands or a fork just until combined. Avoid packing the mixture tightly, which can make the meatloaf dense.
Shape the mixture into a compact oval loaf. In a small bowl, stir together the ketchup and Dijon mustard, then spread the glaze over the top and sides of the loaf.
Arrange the carrots, potatoes, and parsnip in an even layer in the slow cooker. Pour the chicken broth around the vegetables, keeping it off the glazed meatloaf. Set the loaf on top of the vegetables.
Cover and cook on low until the meatloaf is cooked through and the vegetables are tender. Check that the center of the loaf reaches 165°F.
Lift the meatloaf carefully onto a cutting board and let it rest for 5 to 10 minutes before slicing. Serve with the root vegetables and spoon a little cooking broth over them if desired.