Preheat oven to 425
Toss green beans in bowl with 1 tablespoon olive oil and divide into two baking sheets
Roast green beans for 20-25 minutes until tender
Heat 1 tablespoon of oil in saucepan over medium heat
Add onions and cook until soft. Add flour and salt. Add milk and keep stirring until sauce is thick (for about 4 minutes).
Once green beans are done being roasted, preheat oven to broil
Transfer half green beans to baking sheet then cover with half the sauce. Layer other half of green beans and cover with the rest of the sauce.
Combine bread crumbs and remaining oil into a small bowl and sprinkle over green beans.
Place green beans in the oven on broil for 2-4 minutes, watch until breadcrumbs become brown at the top
Let sit for 10 minutes before serving