Make broth: Rinse chicken and set aside to drain. Put coriander seeds and cloves in a dry 6- to 8-qt. pressure cooker. Over medium heat, toast until fragrant, shaking, several minutes. Add onion and ginger and cook, stirring, until browned on edges, 2 to 3 minutes. "Traditionally, the ginger and onion would be charred directly on a burner. I've found that this is the best shortcut to get that flavor," says Nguyen.
Add 4 cups water, then the chicken, breast side up. Add apple, cilantro, salt, and another 4 cups water. "Having only half the water in the pot when the chicken goes in means you won't splash yourself." Lock the lid in place.
Following your cooker's instructions, bring to low pressure (8 psi) over high heat. Lower heat to maintain pressure. Cook 15 minutes, or a few minutes longer if your cooker's low setting is less than 8 psi. If your cooker has only a high-pressure (15 psi) setting, cook 12 minutes.
While broth cooks, soak noodles in hot tap water until pliable and opaque, about 10 minutes. Drain, rinse, and drain well. Divide among four large soup bowls.
When broth is done, lift cooker to a cool burner and let pressure decrease until pressure indicator drops, about 20 minutes. Remove lid.
Let cool 5 minutes, then use tongs to transfer chicken to a bowl. "Don't worry if some parts fall off. Just add them to the bowl too." Add cold water to cover, then soak 10 minutes to cool. Pour off water, partially cover chicken, and set aside to finish cooling.
With a shallow ladle, skim most of fat from broth. Strain broth through a muslin- or cheesecloth-lined strainer into a medium pot, pressing to expel as much broth as possible. Discard solids. Season broth with fish sauce, plus salt and sugar to taste. "Add fish sauce now, at the end. Otherwise you lose some of its umami oomph."
To serve, bring broth to a simmer over medium heat. Fill a medium pot with water and bring to a rolling boil for noodles.
Meanwhile, prepare your pho assembly line: Slice breasts and legs from chicken; set aside half for another use. Cut or tear breast and leg meat into 1/4-in. slices or shreds (discard skin) and put in a bowl. Put red onion, green onion, cilantro, and pepper in separate small bowls. Set out the bowls of soaked noodles.
When water is boiling, tip the first bowl's noodles into a noodle strainer or other small mesh strainer. Dunk strainer into boiling water, stirring noodles with chopsticks or a fork to loosen them, until they're soft, 20 to 40 seconds. "Lift one out and pinch it to see whether it's tender. Some noodles are thicker and need more time." Pull strainer of noodles from water, shaking to drain into pot, and empty into the soup bowl. Top with chicken. "Arrange the chicken so it's flat in the bowl. That way, it gets nicely heated by broth." Add red onion, green onion, cilantro, and a sprinkle of pepper, to give it a little pop.
Taste broth once more and season with fish sauce, salt, and/or sugar if needed. Divide among bowls. Serve immediately, with Garnish Plate and Ginger Dipping Sauce.