Preheat the oven to 425°F. Line a large, rimmed baking sheet with foil.
Add the green beans to the pan and toss them with olive oil, kosher salt, black pepper, and garlic powder. I use my hands to ensure the beans are well coated.
Spread the coated beans in a single layer on the baking sheet.
Roast them until tender and browned in spots, about 15 minutes. Serve immediately.
Video
Notes
As with most vegetables, you should avoid overcooking green beans. You want them to be al dente, just like pasta. When done, they should still have a "bite" and a bit of crunch - you don't want them soft and mushy.
Dry the green beans thoroughly after washing them. Otherwise, they will steam instead of roasting.
I prefer garlic powder to fresh minced garlic because it's easier to evenly distribute. But if you use fresh minced garlic, that's fine.
You can keep the leftovers in an airtight container in the fridge for 3-4 days. I like to eat them cold, like antipasti. But if you wish, you can reheat them uncovered in a 350° oven or covered in the microwave at 50% power. I don't recommend freezing the leftovers.