Puree the garlic, bird's eye chile, lemon grass, fish sauce, and coconut milk in a blender until smooth; set aside. Bring a saucepan of salted water to a boil over high heat. Add the green beans, and cook for 2 minutes; drain.
Heat the peanut oil in a skillet over medium-high heat, and stir in the green onions. Cook and stir 1 to 2 minutes until the green onions have softened and begun to brown. Stir in the pureed sauce, and bring to a simmer. Cook for 2 to 3 minutes, then add the green beans. Reduce heat to medium-low, and simmer the green beans until tender, about 5 minutes.