Char onion and ginger on top of stove, placing them on a piece of wired mesh or a wire rack. You can also char it in a hot cast iron skillet or broil it in the oven for about 15 minutes. Peel off any charred ginger skin or onion skin.
Place pre-boiled bones in pressure cooker, along with tendon, outside flank, toasted star anise and cloves, cinnamon, salt, fish sauce and sugar. Cover with water. Secure pressure cooker cover and cook for 40 minutes at high pressure; quick release pressure, remove cooked tendon and outside flank. Leave bones in the pot. If there is any extra uncooked outside flank, add it to the pot and bring the pressure cooker back up to high pressure and cook 40 more minutes. Quick release pan, remove cooked outside flank and place in of cool water.
Place pre-boiled bones in pot, along with tendon, outside flank, toasted star anise and cloves, cinnamon, salt, fish sauce and sugar. Cover with water to within two inches of top of pot. Bring to a simmer and cook for 1 1/2 - 2 hours until outside flank and tendon are firm but tender; remove outside flank and tendon and place in a bowl of cool water. Continue to cook pho broth for another hour.
Strain broth. Refrigerate pho broth overnight; use a spoon to remove the fat layer. Remove outside flank and tendon from bowl of water, and refrigerate.
If you don't use tendon, double the amount of outside flank.
This makes a lot so just freeze the extra soup and meat in portions that suit your needs.
Adapted from Pleasures of the VIetnamese Table by Mai Pham.