Preheat your oven to 350°F and lightly butter a 9x13 baking dish—this step matters because it prevents sticking and gives you a head start on browning the bottom layer. I confess I've skipped this once, and the casserole stuck like it was glued down, so don't be me.
Melt butter in a large skillet over medium heat, then add your sliced mushrooms and sauté for three to four minutes until they release their moisture and start to soften. This step builds flavor because you're letting the mushrooms caramelize slightly instead of dumping them raw into the sauce where they stay rubbery and bland.
Pour in the cream of mushroom soup and milk, then whisk until the mixture is smooth—no lumps hiding in the corners. Stir in the garlic powder, onion powder, salt, and black pepper, which is where the green bean casserole Thanksgiving sharing recipe stops tasting like every other version at the table.
Add your trimmed green beans and the chicken broth to the sauce, then fold everything together gently for about two minutes so the beans coat evenly without breaking apart. The broth here is your insurance against a dry casserole midway through baking—it keeps the sauce liquid and luxurious as things heat up.
Stir in the shredded cheddar cheese until it melts completely into the sauce, which takes about ninety seconds of stirring. Then fold in half of the fried onions—this is the secret that keeps them crunchy because you're protecting them from the full baking time.
Pour the entire mixture into your prepared baking dish and spread it into an even layer. Bake uncovered for 45 minutes until the edges bubble and the top starts to set, which is when you know the heat has penetrated all the way through.
Remove from the oven and scatter the remaining fried onions across the top in an even layer, then return to the oven for exactly 5 minutes more so they toast without burning. Sandra told me she always checks at the four-minute mark because every oven behaves differently, and that small moment of attention makes the difference between crispy and burnt.