Season the flour with salt and pepper and toss the oxtail in it. Heat the oil in a large casserole dish. Once smoking hot, brown the oxtail all over, then remove from the pan. Add the garlic, herbs and vegetables to the pan and cook for 10-15 minutes, until golden. Add the wine, port and tomato purée and cook over high heat until reduced to a syrupy consistency.
Return the oxtail to the pan with the treacle, peppercorns, star anise and both stocks. Bring to a very gentle simmer, cover and cook for 3 hours, or until the meat is falling off the bone.
Strain the sauce into a clean saucepan, then bring it to a simmer over moderate heat and cook until reduced by half. Pick the meat from the bones into a bowl. Add enough of the reduced sauce to loosely bind the meat together, then mix in the sherry vinegar, 1 tablespoon of the flat-leaf parsley and check for seasoning. Reserve the remaining sauce for serving.
Melt the butter in a saucepan over medium heat until foaming. Add the celeriac and a good pinch of salt and cook for about 5 minutes until the celeariac is golden and tender. Add to the oxtail mixture.
To make the ravioli, brush the outside edges of one of the gyoza wrappers with water. Place a small spoonful of the oxtail filling in the centre and place another gyoza wrapper on top. Press the edges to seal, making sure you remove any air trapped inside. Continue with the remaining wrappers and filling until you have 18 ravioli. Arrange them in a single layer in a steamer set over a pan of boiling water (you may need to work in batches, keeping the cooked dumplings hot while you cook the rest). Steam for 5 minutes.
Heat the reserved sauce and serve with the ravioli and drizzle with olive oil.