Filling: Using a kitchen towel, squeeze off the excess liquid from the spinach. In a medium-sized bowl, mix together the spinach, feta, jalapenos, and yogurt. Set aside.
In a small bowl, mix together the milk and grits. Set aside.
In a large bowl, combine the onion, turkey, eggs, Worcestershire, parsley, stock, salt, pepper, and the grits. Mix until well combined. Transfer half of the mixture to the casserole dish and shape it into a medium-sized log. Using your fingers, make an indentation in the meat for the filling. This should resemble a canoe. Add the filling to the indentation and top with the remaining meat. Form the meatloaf, being sure to cover the filling completely with meat. Pour the tomato sauce over the meatloaf. Bake in the oven until the internal temperature registers 165 degrees F on an instant-read thermometer, about 55 to 60 minutes. Remove the meatloaf from the oven and let it rest for 10 minutes before slicing and serving.