Measure out your stock, vinegar and red wine. Wash the rosemary, slice the shallots and smash the garlic cloves with your fist or the back of a knife.
Put the saucepan on the heat and allow to heat for 2 minutes before adding the olive oil and shallots to fry. Keep stirring as the shallots sauté in the oil ensuring not to burn or over brown them. After 1 minute add the smashed garlic and rosemary and sauté for 1-2 minutes more.
Now add the balsamic vinegar and stir ensuring it doesn’t burn. When the mix is a syrup (2-4 minutes) get ready to add the wine.
Add the red wine stir and allow the alcohol to evaporate (about 3 minutes) and leave to reduce slightly approximately 2 minutes.
Next add the stock and allow to come to the boil, then turn the temperature down to low and reduce by at least half. This will be about 10-15 minutes just keep an eye on it and taste if unsure.
Finally, remove the smashed garlic and rosemary stick and then season to taste with salt and pepper, add the butter and whisk.
Notes
If you want a smooth sauce run the sauce through a sieve to collect the shallots and fallen rosemary leaves. I like to leave mine in because the shallots are simply delicious!