Heat the oven to 375°F and coat a 9-by-13-inch baking dish with cooking spray.
Spread the egg noodles, tuna, mushrooms and peas in the dish, then pour in the broth and milk and add the onion powder and salt.
Stir until all noodles are wet, press them under the liquid and seal the dish tightly with foil.
Bake for 30 minutes, then stir and test a noodle from the center; if it is still firm, reseal and bake for 5 minutes more.
Stir the panko and melted butter together in a measuring cup, scatter the crumbs over the tender noodles and bake uncovered for 8 minutes.
Rest for 5 minutes before serving, when the sauce should be thick but still moist.