Toast the cinnamon, cardamom, cloves, peppercorns, star anise, allspice, fennel, mace, and turmeric in a small heavy saucepan over medium heat for about 2 minutes until fragrant but not darkened.
Roughly slice the ginger. Cut the lemon grass into a few pieces and bruise lightly with a rolling pin or with the back of a knife.
Add the water, ginger, and lemongrass. Bring to a gentle boil, then reduce to a low simmer and cook uncovered for 30–35 minutes, topping up with a little water if needed.
Continue simmering, with all solids still in the pan, until the liquid has reduced to around 450–500 ml. Stir in the brown sugar, maple syrup, and salt until dissolved. The syrup should lightly coat the back of a spoon.
Remove from the heat and add the black tea, vanilla pod and seeds, mandarin zest, and mandarin juice. Cover and let infuse for 10–15 minutes, stirring once or twice.
Strain through a fine sieve or muslin into a clean jug, pressing gently to extract all the liquid. Adjust consistency with a splash of water if too thick, or balance sweetness or acidity to taste.
Pour into a sterilised glass bottle or jar, seal, and refrigerate for up to 4 weeks.