This is based on a traditional recipe of my mother's - we'd see them Thanksgiving and Christmas, but because I loved seafood I've come up with variations. Depending on individual likes, the door's wide open for creativity.
I find it best to buy loose mushrooms so the biggest ones can be selected for stuffing. I have also bought packages of them if I decided there were enough bigger mushrooms for my purposes.
Heat oven to 350.
Wash mushrooms. Remove stems and set aside. (These will be diced and used in the stuffing mixture.) Set washed mushrooms aside.
Prepare the stuffing mix. Traditionally, no woman in my family used a measuring cup, we used handfuls, palmfuls, pinches, sprinkles, so I have to think how to tell you how much. Depending on how many mushrooms you'll be stuffing, pour out 1/4 to 1/3 of the breadcrumb container into a mixing bowl. Add garlic powder and basil to taste - for the basil I'd say pour some into the palm of your hand, press hands together, hold them over the mixing bowl, grind with your palms and let basil sprinkle into the breadcrumbs. Decide from there if you want more.
Take the stems, finely chop and add to seasoned crumbs. At this point finely chop any seafood you'll be adding, mix all ingredients around with a wooden spoon, then add some warm water and a drizzle or two of olive oil to make into a pasty substance. (I mix using my hand. If that gives you the willies use a wooden spoon, but by hand tells you firsthand if the substance is the right consistency for stuffing purposes.)
Spray a baking pan with your favorite no-stick spray. Begin stuffing mushrooms using a teaspoon or pinch amounts with your fingers; pat stuffing into mushroom, place in pan. Line pan with stuffed mushrooms. When you're finished, add a very small amount of water to the bottom of the pan, maybe 1/8 of an inch worth's, drizzle olive oil over all the mushrooms, cover with foil to bake.
Bake for 45 minutes to 1 hour at 350. (Depending on your oven, it may take more time or it may take less.)