I questioned, in my first version of this stew, whether you needed to soften the onions first. I have since decided that a gentle frying/stewing in oil for 10 to 15 minutes gives them a sweetness (eliminating their acidity) that may enhance the dish.
I made the stew again yesterday. Once the onions had softened, I added the herbs, star anise, half a tsp of salt, plenty of ground black pepper (ground pepper is reputed to turn bitter in stock, but does not seem to do so in stews, in my experience), and the oxtails. No stock or water.
After about an hour in the even at a low heat, the stew was creating its own sauce. At this stage, I added the tbsp of ketchup and the soy (which might have caught and burned when the casserole dish was dry).
I gave the stew three and a half hours in total. As before, I removed the oxtails to a board, sieved the sauce – of which there was just the right quantity - into a small saucepan (pressing down on the vegetables). Then I returned the oxtails and sauce to the casserole, covered it, and put it back in the oven for 10 minutes or so, to warm through.