Trim the stem ends off the green beans, rinse, and dry them thoroughly with a towel.
Heat the olive oil and 1 tablespoon of butter in a large skillet over medium-high heat until the butter foams.
Add the green beans in a single layer and sear undisturbed for 2 to 3 minutes until lightly charred in spots.
Stir, season with salt, pepper and red pepper flakes, and cook 3 to 4 minutes more, tossing often, until crisp-tender.
Add the water, cover for 1 minute to steam, then uncover and cook off any remaining water.
Push the beans aside, add the remaining butter and the sliced garlic, and cook 30 seconds to 1 minute until fragrant.
Remove from the heat, toss with the lemon juice and zest, garnish if using, and serve immediately.