Set the oven to 400 degrees Fahrenheit.
Turn the pot’s heat back to medium-high and add the oxtail together with all the remaining ingredients except the browned baby onions and the mushrooms. Give the whole lot a good stir to combine everything, and as soon as it comes to boil turn off the heat. Give it a final stir, cover the pot with a sheet of heavy tin foil that comes about 1 1/2 inches down the sides of the pot and put on the lid.
The pot now goes into the oven at 400F. After 20 minutes, turn the heat down to 340F. Then, after another two hours at 340F, remove the pot from the oven, take off the lid and the foil, and add the browned baby onions and the sliced mushrooms. Give the pot a stir to mix in the onions and mushrooms. Also, now’s the time to taste the sauce and check if you think it’s salty enough. If not adjust to your taste.
And while you’re there, if the stew’s perhaps looking as if it could do with some more liquid, well, why not add some more wine. Certainly won’t do any harm.
Return the pot – this time uncovered – to the oven for another hour at 340F, then turn off the oven. Done.