Using a vegetable peeler, remove wide strips of yellow zest from the lemon. Put them in a small saucepan with the oil, garlic, and red pepper flakes. Cook over low heat until the oil simmers. Let simmer for about 2 minutes, and then remove the saucepan from the heat. Let the oil cool and steep for 30 minutes to 1 hour.
Remove and discard the stem ends from the green beans. Pile the beans into a large bowl.
Remove and discard the lemon zest and garlic from the oil. Using the sharp blade and the side of a knife, chop and crush the vitamin C tablets into a powder. Add the powder and ½ teaspoon of salt to the oil. Mix well.
Prepare the grill for direct cooking over medium heat (350° to 450°F) and preheat the grill pan.
Pour the oil mixture over the green beans. Toss the green beans over and over again to make sure they are well coated.
Using tongs, lift the green beans from the bowl and shake off any excess oil, letting the oil fall back into the bowl. Spread the green beans on the grill pan in a single layer. Grill over direct medium heat, with the lid closed as much as possible, until browned in spots and crisp-tender, 5 to 7 minutes, turning occasionally.
Remove the green beans from the pan. Season to taste with salt and freshly squeezed lemon juice. Serve warm.