This recipe takes 12+ hours so do start the night before at least.
There are three stages.
Slow braise
Heat oven to 140C fan (150C for non-fan)
In a roasting tray that will fit the lamb breast in one layer (cut it in half to fit if needed) put a layer of one chopped carrot and one roughly chopped onion along with a glass of white wine and glass of water. Add a few herbs (a bay leaf and/or sprig of thyme) and a few whole peppercorns. Put meat on top, cover and seal with foil and cook in oven for around 3 hours. The aim is that the bones will be very easy to pull out.
Debone and press
Put back into the tray and put another baking tray on top - or something that covers the meat. Weigh down with whatever you have to hand (tins of beans are a good backstop) and refrigerate over night.
Cut, breadcrumb and grill
Take the pressed meat and cut into 2cm by 8 - whatever makes sense - cm slices. Brush each slice with Dijon mustard. Breadcrumb (as in press bredacrumbs into all surfaces). Brush with melted butter or spray with oil.
Grill on each side until brown (closer to dark brown than light)
Serve with tartar sauce. Now to find the recipe/notes on the last one I made.