Bring a large pot of water to a boil and add 1 teaspoon of salt. Add the green beans and cook for 2 minutes.
Drain them and quickly place them in a bowl with ice-cold water to stop the cooking process.
Heat the oil in a large skillet over medium heat. Add the onion and garlic, cooking until softened and fragrant.
Add the green beans, season with salt and pepper, and sauté for about 5 minutes until they reach your preferred tenderness.
Stir in the butter, mustard, and lemon juice if using them. Check for seasoning and adjust as needed. Serve the green beans warm and enjoy the blend of flavors.
Notes
Keep Beans Crisp: Do not overcook the beans during blanching to maintain their crisp texture.
For more zest, try adding crushed red pepper flakes during the sauté.
Fresh beans have more snap, but frozen beans are convenient year-round.