Pre heat oven to 400 degrees. Line a rimmed baking sheets with foil. Set aside.
In a very large bowl, combine turkey, panko bread crumbs, milk, eggs, Worcestershire sauce, and ketchup. Mix well using very clean hands- I find this is the best way to mix. Mix by lifting and folding the mixturefrom the bottom of the bowl up and over. Handle the turkey mixture gently and do not overpack.
In a large pan over medium high heat, add the oilve oil . Add in the onions and sauté for 2-3 minutes until glistening and softened. Add in the mushrooms and zuccini and cook until tender. Add in the rosemary, sage, mustard, salt, pepper, fennel, red pepper flakes and garlic powder. Stir well to coat and bloom (sauté)the spices- about 3-4 minutes.
Let the vegetable mixture cool for 10 minutes. Once cooled, add it to the turkey mixture stirring very gently to fully incorporate all the ingredients.
At this point you can cover and chill the mixture overnight to better enhance the flavors.
If baking immediately, form the mixture into a loaf and place on the prepared baking sheet. Place 2-3 sage leaves on top for a garnish.
Bake at 400 degrees for 45 minutes or until the internal temperture registers 170 degrees.
Let rest for 5-10 minutes. Serve the meatloaf in slices with a generous dollop of dijon mustard, your favorite streak or BBQ sauce.