For the marinade, stir together fish sauce, garlic, 5-spice powder, lemongrass, and pepper.
Put beef shank and tendons in a large bowl, add marinade and toss to coat. Let marinate for at least 60 minutes, or longer if time permits.
Program an 8-quart programmable IP to Sauté. Add oil making sure to coat the bottom. Sear the beef in two batches until all sides are brown, about 5 minutes, adding more oil between batches as needed. Transfer seared beef to a plate. Reserve any leftover marinade in bowl.
Add shallot, ginger, garlic, lemongrass, and star anise to pressure cooker. Cook stirring often, until fragrant, about 3 minutes.
Add tomato paste, sugar, Hoisin sauce, and salt.
Return seared beef, tendons, accumulated juices and any reserved marinade to cooker. Stir to combine, and add 6 cups of water to cover the solids.
Press Cancel button to reset cooking program. Lock lid. Select Pressure Cook at high pressure for 20 minutes. Manually release the pressure from cooker until float valve drops.
Unlock lid. Beef and tendon should be chewy-tender but still have some bite. Skim off fat, if needed, and add carrots.
Lock lid. Select Pressure Cook at high pressure for 3 minutes. Manually release the pressure from cooker until float valve drops.
Taste and season with additional fish sauce or sugar to your taste. If it's too salty, add water to lighten it.
To make the dipping sauce, in a small bowl mix all the ingredients together.
To serve, ladle beef stew into individual bowls. Garnish basil, sawtooth, and a sprinkle of freshly ground pepper. Serve with salt/lime/chili mixture and toasted baguette.