Melt the butter in a pan, add the olive oil and sugar and stir to dissolve the sugar.
Add the shallots and cook for a couple of minutes, stirring. You might notice a couple of sugar lumps - just leave them there.
Hit the pan with the vinegar, give it all another good stir, cover, lower the heat to minimum and cook for 25-30 minutes. If the liquid evaporates, add a little water.
Remove the lid and simmer over medium to high heat for another 3-5 minutes. You’ll end up with a kind of a thick, luscious shallot stew.
While the shallots are cooking, fry the potatoes in a mix of oil and butter until golden and soft.
Prep the steaks - take them out of the fridge at least half an hour before cooking and season with salt, pepper, dry rosemary and olive oil.
When the shallots and the potatoes are ready, heat a grill pan until hot and cook steaks for a few minutes on each side. It takes 2 minutes on each side plus resting time for a 1.5cm steak to achieve a perfect medium.
Let rest for 3-5 minutes before slicing and serving.
To serve, slice each steak and arrange on a plate together with potatoes. Add a generous topping of braised shallots and decorate with a sprig of rosemary.