Crush shallot, garlic, lemongrass, and ginger in a mortar and pestle to a coarse paste; set aside. (If you don’t have a mortar and pestle, chop very finely.)
2
Heat oil in a large skillet over medium-high. Arrange green beans in a single layer; cook, undisturbed, until browned underneath, about 3 minutes. Using tongs, turn beans over and cook, tossing occasionally, until deeply browned in spots and tender, about 5 minutes. Add reserved lemongrass mixture; cook, tossing, until fragrant and tender, about 1 minute. Remove from heat. Add oyster sauce, season with salt, and toss to coat.
3
Transfer green beans to a platter and top generously with crispy onions.
