Put flank steak in the freezer for 30 mins-1 hour to make it easier to slice.
While flank is freezing, place the onion and ginger on a small roasting pan and broil for 20-25 minutes, turning occasionally until charred on all sides
Allow onion and ginger to cool, then peel off outer charred layer.
After meat is chilled, slice into very thin strips
Heat oil in a dutch oven on medium heat.
Cook garlic and lemongrass until soft, about 5 minutes
Add onion, ginger, stock, water, soy sauce, fish sauce, sugar, star anise, cloves, tabasco and cinnamon and bring to a boil
Reduce heat to a simmer and cook, covered for 20 minutes.
Strain and discard the solid ingredients, reserving the broth.
Return the broth to low heat and add the sliced beef to the pot.
The pho is ready when the meat is no longer pink (the heat from the broth cooks the meat very quickly)
Divide the noodles, evenly between 6 bowls, ladle the pho on top and add your favorite toppings for garnish.