Melt the butter in Instant Pot on "Sauté" setting, add pieces of beef shank in batches, browning the pieces, then setting aside. Add garlic and the one onion cut in large pieces, stirring to cook until browned and onion softens. Add Shaoxing and stir. Make a packet using a piece of cheesecloth, and wrap the bay leaves, star anise, cinnamon stick, and lemongrass pieces, tying tightly. Add packet to the pot with the ginger, tomato paste, and stock. Stir well until tomato paste has mostly dissolved. Add carrots and potatoes and place the meat back in the pot, ensuring it is covered in liquid. Close Instant Pot, sealed on "Meat/Stew" setting and cook for 50 minutes. Release pressure, add fish sauce, stirring well. Ladle into bowls and serve with thinly sliced onion, basil, cilantro, lime pieces and baguette.