Rehydrate dried mushrooms in 1 1/2 cups of scalding water for at least 30 minutes. Strain the mushroom rehydrating liquid into a measuring cup and mix (as needed) with broth to make 1 1/2 cups. Remove the rehydrated mushrooms and rinse. Chop finely.
Meanwhile, in a heavy-bottomed pot, melt the butter over medium heat. Add the mushrooms, salt, and pepper. Cook, stirring frequently, until the mushrooms have given up their liquid. Continue cooking until liquid is evaporated. Add the garlic and thyme; cook for ~1 minute more. Sprinkle flour over the mixture; stir to combine. Slowly stir in the rehydrating liquid (a tablespoon at a time), allowing each addition to absorb completely before adding more. (This will prevent lumps, so be patient!) Stir in the cream and wine; bring to a boil. Simmer, stirring occasionally, until the sauce is thickened (~15 minutes). Add the marsala (if using); stir. Simmer for 2 additional minutes.
You can do all of the above and freeze the dish; thaw three days before serving. Otherwise, make the dish up to two days before serving and keep in the refrigerator.
Make the topping by mixing the melted butter with the panko, fried onions, and S&P. Unwrap the dish, discard plastic wrap. Sprinkle the topping evenly over the casserole; re-cover with foil. Allow the casserole to sit at room temperature for about an hour before it goes in the oven. Bake the casserole at 350° for 20 minutes, covered. Remove foil; bake an additional 10 minutes or until topping is golden-brown.
*If you'd like, you can chop up an additional cup of the fried onions and mix them in with the beans and sauce to kick the flavor up even more. However, I don't recommend this strategy if you're planning to freeze the casserole.