Preheat your oven to 350°F.

In a frying pan over medium-high heat, sauté the mushrooms in butter. Add salt to help extract the moisture. Once halfway through, add the onions.

Stir in the garlic towards the end to avoid burning and bitterness.

Once the mushrooms have cooked down and reduced, set them aside to cool.

Prepare the gravy mix according to the package instructions and set aside.

In a large bowl, thoroughly mix the ground beef, eggs, and panko breadcrumbs.

Incorporate half of the cooled mushroom and onion mixture into the meat mixture.

Line an 8x8 cake pan with parchment paper, or use any other baking dish of your choice. Pack the meat mixture tightly and then transfer it onto a parchment-lined baking sheet to maintain its shape.

Bake the meatloaf for about 1 hour. Check with a thermometer to ensure the internal temperature reaches 160°F, adjusting cooking time as needed based on loaf thickness.

15 minutes before finishing, mix the remaining mushroom and onion mixture with 2-3 tablespoons of the gravy to a thick, spreadable consistency.

Spread this mixture over the meatloaf and top with Swiss cheese slices, overlapping if necessary.

Continue baking until the cheese melts.

Remove from the oven, let it rest for 5 minutes, then slice and serve with the remaining gravy.