Heat a large pot of lightly salted water to a boil. Add green beans and cook at a gentle simmer until just tender, about 8 minutes. Drain well.
Meanwhile, gather remaining ingredients and melt butter in a medium skillet. Add garlic and red pepper flakes and saute on low heat until fragrant and slightly softened, about 1 minute, being careful not to brown. Whisk in lemon juice, Dijon, salt and pepper.
Add cooked beans and toss until evenly coated with butter sauce. Serve immediately.
-Green beans can be cooked and cooled in an ice bath up to one day in advance. They will need more time to warm in the butter sauce if adding from a chilled state.