Preheat the oven to 400 F. Coat a baking sheet with cooking spray or vegetable oil.
Spread green beans in single layer on baking sheet. Pour olive oil over beans, sprinkle liberally with salt and pepper. Toss to coat the beans with oil.
Roast in the center rack of the oven for 20-25 minutes, until the green beans are just starting to brown.
In a small pan, melt the butter over medium heat. Add lemon slices to butter and cook for 2 to 3 minutes, until lemons start to brown around the edges. Add garlic and cook for1 minute until the garlic turns golden. Remove from the heat.
Plate the green beans to a serving dish and pour the sauce over the green beans. Serve immediately.