Preheat oven to 425 degrees F. Prepare a 9x13 baking dish with cooking spray (or butter).

Toss green beans in a large bowl with 1 tablespoon oil until well coated. On a large baking sheet (or 2 baking sheets) spread in an even layer. Roast, stirring once until tender and beginning to brown, 20 to 25 minutes. If using 2 pans, rotate the pans top to bottom about halfway through.

Meanwhile, heat 1 tablespoon oil in a large saucepan over medium heat. Add onion and cook, stirring frequently, until very soft and golden brown, 2-4 minutes. Add another tablespoon of oil and the leeks and mushrooms and cook for another 6-8 minutes.

To the onion mushroom mixture, add flour, salt and pepper; cook, stirring, for 1 minute more. Pour in milk and continue to stir, scraping up any browned bits. Cook, stirring, until the sauce bubbles and thickens enough to coat the back of a spoon, about 4 minutes. Add in the 1/2 cup of cheese until it melts.

When the green beans are done, remove from the oven. Preheat the broiler.

Transfer half the green beans to a 9x13 baking dish. Spread half the sauce over the green beans. Add the remaining green beans and top with the remaining sauce.

Combine breadcrumbs, the remaining 1 tablespoon oil, and 1/4 cup shredded cheese in a small bowl.

Sprinkle the breadcrumb mixture over the gratin. Place under the broiler and broil, watching closely, until the gratin is bubbling and beginning to brown on top, 1 to 5 minutes, depending on your broiler. Let stand for 10 minutes before serving.