Preheat the oven to 350 degrees F. Set out a rimmed baking sheet (no need to grease it).
Place the frozen spinach in a microwave-safe bowl and thaw it on high. Do not drain the spinach, as it will help add moisture to the meatloaf. While the spinach is thawing, peel and finely chop your garlic.
Shred or grate your cheddar if starting with a block. Use a blender or food processor if you have one. Set aside the cheese to be used later.
Add the spinach, garlic, oats, eggs, Worcestershire, seasonings and ground meat to a large mixing bowl. Use clean hands to gently mix everything together until the spinach, oats, eggs and seasonings are distributed evenly.
Transfer the meatloaf mixture to your rimmed baking sheet. Shape the meat into a large rectangle. It should nearly reach the top and bottom edges of the pan. Use your hands to make a rectangular well or indentation in the meat, starting from a couple inches inside the edges. Place your cheddar inside the borders of the well.
Starting at the outside edges, gently push the meat toward the center, covering the cheese and enclosing it completely. Pat the meatloaf into a level rectangle, so it will cook evenly. Wash your hands well.
Bake at 350 degrees F for one hour, or until the meat is no longer pink.
While the meatloaf is baking, make the potato gravy. Peel your potatoes and cut them into cubes. Cook them in the microwave, in a microwave-safe bowl, on high for about 8 minutes, stirring halfway. Potatoes are done when you can easily insert a fork in them. Place the cooked potatoes and 3/4 cup chicken stock in your blender and mix until smooth.
In a saucepan, melt the butter over medium-high heat and whisk in the flour, stirring for a minute. Add the milk, salt, pepper and onion powder and whisk constantly, until the mixture starts to thicken and get bubbly. Cook for another minute and then add the potato mixture, stirring to combine. Add the remaining 3/4 cup of chicken stock. Continue stirring until the gravy is mixed and heated. Turn off the heat. You can reheat the gravy for a few minutes when the meatloaf comes out of the oven.
When the meatloaf is done, let it rest at room temperature for 5-10 minutes before slicing, to give the cheese a chance to set a bit. (Otherwise, it will run out as soon as you slice.) You can reheat your gravy at this time, if needed.
To serve, spoon some potato gravy onto each slice of meatloaf. Store leftover meatloaf and gravy separately in the refrigerator for up to four days.
This recipe can easily be cut in half, but I like to make enough for leftovers.