This is so easy and delicious, especially on a cold night. Here's the original recipe, which I think I follow.
In a small bowl, combine 1/2 pound ground pork (or less if needed) with 2 T fish sauce and 2 tsp sugar.
In a big pot, throw in 2-4 smashed stalks of lemongrass (I buy this in large quantities at Asian markets since it's very expensive at most grocery stores and it freezes well), two 1-inch pieces of smashed ginger, and 12 cups of water. Bring to a boil and boil for 5-10 minutes, then add 1 1/2 cups rice and stir till the water boils again. Lower the heat and gently boil for 15-20 minutes till the rice is tender. Remove from the heat. You can discard the lemongrass and ginger if you want.
While the soup is cooking, heat a bit of oil in a skillet and add 10 cloves of minced garlic. Cook briefly, then add the pork and cook till the meat has changed color. Dump into the soup. If the soup has been sitting long, heat it back up.
Top with well-washed bean sprouts, fried shallots, fish sauce, cayenne, and rice vinegar if you like.