Bring a large saucepan of water to a boil over medium high heat, and fill a large bowl with ice and water. The ice bath needs to be ready first so the beans can stop cooking the moment they come out of the pot.

Drop the green beans into the boiling water and cook for 3 to 3 1/2 minutes, or 2 to 3 minutes for thin French-style beans. Lift them out with tongs and transfer them straight to the ice bath when they are bright green and still have a little snap.

Melt the butter in a large skillet over medium heat, then add the slivered almonds and cook for about 3 minutes. Watch closely for a light golden color and a nutty smell because almonds can go from toasted to burnt very quickly.

Stir in the garlic and red pepper flakes (optional) and cook for 30 seconds. The garlic should smell fragrant but not darken much, or it can turn bitter.

Drain the beans well in a sieve, then add them to the skillet and toss for 1 minute to coat and warm through. The pan should look glossy rather than watery, and the beans should stay vivid green.

Add the lemon juice, season with salt and pepper to taste, and serve right away. The finished dish should taste buttery and nutty first, with a bright lemon lift at the end.