Melt the butter very gently in a small saucepan. As soon as it is melted (you do not want it to brown), add the sliced shallots in the pan. Cook on a gentle heat (let them sweat- they should not brown) for about 10 minutes.
Pour the red wine into the saucepan. Leave to cook for 20 to 30 minutes on a gentle heat. You need the red wine to have almost all evaporated.
Pan fry your beef bavettes as you like them cooked.
Remove your bavettes from the frying pan. Add the red wine shallots comfit to the pan to get all the flavour from the meat and serve the comfit on top of the meat.