Preheat your oven to 375°F (190°C). In a large mixing bowl, combine the Panko breadcrumbs and whole milk. Let sit for 5 minutes until it forms a thick paste.

Add the ground beef, lightly beaten eggs, diced onion, minced garlic, 2 tbsp Worcestershire sauce, salt, and pepper to the breadcrumb mixture. Gently fold together with your hands until just combined. Do not overmix.

Transfer the beef mixture to a parchment-lined rimmed baking sheet. Shape into a free-form loaf (about 9x5 inches). Bake for 55 to 65 minutes until the exterior is crusty and browned, and the internal temperature reaches 160°F (71°C). Let rest for 15 minutes before slicing.

While the meatloaf rests, heat a heavy-bottomed skillet over medium-high heat. Add the olive oil and 1 tbsp of butter. Add the sliced cremini mushrooms in a single layer and let sizzle undisturbed for 3-4 minutes. Toss and cook 2 more minutes until tender, then remove to a plate.

Reduce the heat to medium. Melt the remaining 3 tbsp of butter in the skillet. Sprinkle in the all-purpose flour and whisk constantly for 2-3 minutes until lightly golden and aromatic.

Slowly pour in the beef broth while vigorously whisking to prevent lumps. Stir in 1 tbsp of Worcestershire sauce. Simmer gently for 5-7 minutes until the gravy reduces and thickens into a glossy, dark brown sauce.

Fold the sautéed mushrooms back into the gravy to warm through. Carve the rested meatloaf into thick slices. Serve on a plate, generously spoon the hot mushroom gravy over the meat, and sprinkle evenly with finely chopped fresh green parsley.