Preheat oven to 400F.
Combine cubed, crustless, bread with milk and allow to soak for about 5 minutes, or until the bread is saturated. Drain & save the milk - I use a sieve and press most of the milk out of the bread with the back of a spoon.
While soaking bread, place sliced mushrooms in a food processor and pulse until finely chopped (see photo). This usually takes about 10-12 one second pulses.
Place the mushrooms in a nonstick pan with a pinch of salt (no oil or butter), and saute them until cooked and almost dry, stirring occasionally.
Beat eggs, mixing in the (recovered) milk, salt, and pepper. Place 2 tablespoons of butter in a hot pan, then scramble the eggs until barely done.
Add the drained bread to the eggs and mix well, chopping up the bread and egg curds with your spatula as you mix, to get the ingredients well incorporated. Add the cooked mushrooms, and mix them in as well.
Place the mixture into a casserole dish and smooth out the top (I use the back of a spoon). Add thin slices of Swiss cheese to cover the top (one well-covered layer is enough).
Melt the remaining 2 tablespoons of butter, and mix with the panko breadcrumbs. Cover the Swiss cheese with the breadcrumbs and smooth the layer out, then sprinkle the garlic powder evenly on top of the completed dish.
Bake at 400 degrees for 15-20 minutes (since the interior of the dish is already cooked, just bake until the breadcrumbs reach a golden brown color, and you are done!)
Let cool for a few minutes, and enjoy.