Preheat oven to 425 degrees.
Toss green beans in a bowl with one tablespoon of olive oil, then divide evenly into two baking sheets.
Roast green beans in the oven for 20-25 minutes, or until they become tender.
While the green beans are roasting, grab a saucepan and heat one tablespoon of oil over medium heat.
Add onions and cook until soft. Then, add flour and salt, followed by milk. Keep stirring the sauce for about 4 minutes, or until it thickens.
Once the green beans are done roasting, preheat your oven to broil.
Transfer half of the green beans to a baking sheet, then cover them with half of the sauce. Add the remaining green beans and cover them with the rest of the sauce.
In a small bowl, combine bread crumbs and the remaining oil, then sprinkle the mixture over the green beans.
Place green beans in the oven and broil for 2-4 minutes. Keep a close eye on to make sure the breadcrumbs become brown on top.
Let sit for 10 minutes before serving. Enjoy!