Toast the Sichuan peppercorns in a dry frying pan until fragrant, then use a mortar and pestle to grind to a rough powder. Set aside.
Place the ginger, garlic, shallots, doubanjiang and 150ml (5 fl oz) of the grape-seed oil in a food processor and blitz to a smooth paste.
Heat 100ml (3½ fl oz) of the remaining grape-seed oil in a large heavy-based saucepan over a high heat. Add the paste and fry, stirring occasionally, for eight to 10 minutes until dried and fragrant, then reduce the heat and simmer for a further 15 minutes, until the rawness from the vegetables has been completely cooked out. Stir in the shaoxing wine, sugar, tamari and one-third of the ground Sichuan peppercorns, then spoon the mixture into a large bowl.
Wipe out the pan, add the sesame oil and remaining two-and-a-half tablespoons of grape-seed oil and heat over a high heat. Working in two batches, add the tuna mince to the pan and fry for two minutes, or until the mince is coloured and has separated into individual strands, then stir through the fried paste mixture to combine.
To assemble the mapo tofu, return the tuna mixture to the saucepan and warm through over a low heat. Add the tofu, cover with a lid and heat for three minutes, then spoon into serving bowls. Top with the spring onion, sesame seeds, chilli, if using, and the remaining ground Sichuan peppercorns.
Serve with steamed rice, if you like.