Heat the oven to 375°F and line a rimmed sheet pan with parchment. Cook mushrooms and onion in butter in a large skillet over medium-high heat for 10 to 12 minutes until released liquid evaporates; cool for 5 minutes.
Soak panko in milk for 5 minutes. Stir in eggs, Worcestershire, thyme, salt, pepper and cooled mushroom mixture.
Fold in beef and shape a 9-by-5-inch loaf about 2 1/2 inches high. Bake for 55 minutes.
Check the center and continue baking as needed until it reaches 160°F, about 5 to 10 minutes more.
Lay Swiss slices over the top and return to the oven for 3 to 5 minutes, just until melted.
Rest for 10 minutes, cut into 8 slices and divide the melted cheese cap among portions.