Preheat the broiler. Place oven rack 4 to 5 inches from the heat source.
Spread the green beans evenly on a baking sheet. Add lemon slices, drizzle with olive oil and salt, and toss well.
Broil green beans for 6 to 7 minutes, tossing and flipping once in between. Transfer to a plate, season with the red pepper flakes or chile piquín, and let each guest squeeze on as much of the remaining lemon as they would like.