Slice beef into long thin strips 1/8 inch by 1 inch by 5 inches long.
Combine the garlic, lemongrass, coriander, sugar and fish sauce in a mortar and pestle until mixture is a paste. Use a blender or food processor if you have to. Marinate meat for 12 to 24 hours in this mixture.
Make Peanut Sauce: Heat the oil in a small pan and add the garlic, chili paste and tomato paste. Fry until the garlic turns light golden brown. Add the broth, peanut butter, hoisin and sugar, and simmer for 3 minutes. Cool and add the peanuts and chilies. Set aside.
Thread beef onto skewers, grill briefly to medium rare over high direct heat and serve with the sauce for dipping, garnishing with the nuts.