Step one: Make the broth
Char, peel, and prep the ginger and onion: Use medium heat on a gas or electric coil burner, medium-hot heat on an outdoor grill or barbecue, or the broil setting in an oven (have the rack in the top or second position). Regardless, let the skin get a little splotchy with black; use tongs to occasionally rotate the ginger and onion (or shallot) and to grab and discard any flyaway skin. When working indoors, turn on the exhaust fan and open a window. To steady the aromatics on the stove, use a small grilling rack, heavy-duty broiling rack, or oven-safe cooling rack.
Monitor the aromatics because they char at different rates due to their uneven size and shape. After 10 to 15 minutes, they’ll have softened slightly and become sweetly fragrant. Bubbling at the root or stem ends may happen. You do not have to blacken the entire surface. Remove from the heat and let cool for about 10 minutes.
Remove the charred skin from the cooled onions or shallots, and as needed, rinse under running water to dislodge stubborn dark bits. Trim off and discard the blackened root and stem ends; halve or quarter each and set aside.
Use a vegetable peeler or the edge of a teaspoon to remove the ginger skin. Rinse under warm water to remove blackened bits. Halve the ginger lengthwise, cut into chunks, then bruise lightly (use the broad side of a knife or a meat mallet). Set aside to add to the stockpot.
When shopping, select firm, solid onions or shallots. If using shallots, big ones endure the charring best. Choose ginger that’s relatively straight; side knobs and little branches make it harder to char and peel. To avoid feeling rushed, char and peel the aromatics a day in advance and refrigerate; cut and bruise before using. Set aside with the apple, celery, carrot, and cabbage.
Put the star anise, cinnamon, fennel, and coriander in a small stockpot, about 8-quart (8 l) capacity. Over medium heat, toast the spices for several minutes, shaking or stirring, until fragrant. Add 4 cups (1 l) of the water to arrest the cooking process. Add the ginger and onion along with the apple and vegetables, salt, and dried shrimp. Pour in the remaining 5 cups (1.25 l) water and the clam juice.
Partially cover and bring to a boil over high heat. Meanwhile, peel and devein the shrimp. Toss the shrimp shells into the stockpot. Cover and refrigerate the shrimp for later use.
When the broth reaches a boil, uncover and lower the heat to gently simmer for 1 hour. (There’s usually no scum to skim.) At the 50-minute mark, add the reserved shrimp to the pot; use a noodle strainer or mesh sieve and work in batches to retrieve them easily. Cook until just curled into a C shape, then transfer to a bowl to cool. If not serving soon, cover and refrigerate.
When the broth is done, let rest for 5 to 10 minutes, uncovered, before straining through a muslin-lined mesh strainer positioned over a medium pot. Press and squeeze on the solids to expel extra broth. Discard the solids. There should be about 8 cups (2 liters).
If using the broth right away, season with fish sauce and, if desired, with extra salt and sugar (or maple syrup). When cooking in advance, partially cover the unseasoned broth, let cool, then refrigerate for up to 3 days or freeze for up to 3 months; season before using.