In a large bowl, combine 1 pound of ground beef with 1 cup of panko breadcrumbs, ½ cup of whole milk, and 1 large egg. Add in 1 finely diced yellow onion, 2 minced garlic cloves, 1 teaspoon of dried oregano, 1 teaspoon of kosher salt, and ½ teaspoon of black pepper. Mix gently until all ingredients are just combined, taking care not to overmix to keep the meatloaf tender.

In a separate bowl, blend 1 cup of crumbled feta cheese with 2 cups of finely chopped fresh baby spinach. If you prefer some heat, mix in a pinch of red pepper flakes.

Preheat your oven to 375°F (190°C). In a greased baking sheet or loaf pan, spread half of the meat mixture evenly across the bottom. Create a well in the center and spread the spinach feta filling over the meat, then layer the remaining meat mixture on top.

In a small bowl, mix together ½ cup of prepared ketchup, 1 tablespoon of Dijon mustard, and 2 tablespoons of light brown sugar until smooth. Spread this glaze over the top of the assembled meatloaf.

Transfer the meatloaf to the preheated oven and bake for 50-60 minutes, or until the glaze is bubbly and an instant-read thermometer reads at least 160°F (71°C).

Remove the Spinach Feta Stuffed Meatloaf from the oven and let it rest for about 10 minutes.