In a blender, blend garlic, ginger, lemongrass, black pepper, and grapeseed oil until smooth. Pour over flank steak and let marinate for at least one hour in the refrigerator.
For the spicy lime dressing, blend all ingredients in the blender at high speed. Keep dressing cold in the refrigerator.
Remove skirt steak from refrigerator and let it sit at room temperature for about 30 minutes. Before grilling steak, remove excess marinade by blotting with paper towels. Season the steak generously with salt and additional freshly ground black pepper. Grill skirt steak over high heat, about 3-4 minutes on each side for medium-rare. Remove steak from grill and let rest at least 5 minutes. Meanwhile, compose salad of honeydew, watercress, avocado, red onion and basil on serving plates. Dress generously with spicy lime dressing. Slice steak thinly against the grain and plate with salad. Serve immediately.