Preheat your oven to 350°F and grease a 9x13-inch baking dish with the olive oil. This step matters because a dry pan causes the edges to stick and forces you to scrape instead of transfer smoothly.

Heat the melted butter in a small bowl, then whisk in the garlic powder, salt, and black pepper—let it sit for two minutes so the garlic blooms. This foundation is what separates a forgettable casserole from one people ask for the recipe on, because garlic needs those moments to release its flavor.

Drain your canned green beans thoroughly in a colander, pressing gently to remove excess liquid—don't skip this step. Watery green beans turn your sharing thanksgiving dish into soup, which is the number-one mistake I see people make.

In a large mixing bowl, combine the cream of mushroom soup with milk and whisk until smooth, then fold in the drained green beans. I always taste here to check the salt level before moving forward because canned soup varies in sodium content.

Pour the butter mixture into the soup mixture and stir well, then fold in half the fried onion strings and the cheddar cheese. Hold back the remaining onions for topping—this gives your crowd casserole texture on every level, not just the surface.

Transfer the mixture to your prepared baking dish and spread it evenly with a rubber spatula. Uneven thickness causes the edges to overcook while the center stays slightly soft, so take your time here even though it feels tedious.

Bake for 40 minutes until the filling begins to bubble at the edges, then remove from the oven and top with remaining fried onion strings. This timing keeps the onions from burning while they toast to golden brown.

Return to the oven for 10 more minutes until the onions are crispy and the filling bubbles gently throughout. You'll know it's ready when the aroma becomes almost unbearable—that's your signal.