Just before Xmas I was lucky enough to spot an Onglet at my butcher's.
I first had onglet in a small french restruant in Nottingham, I ordered it because the menu said "This steak will not be served any more cooked than medium rare, if you like a more well done steak please feel free to order something else"(*). Which to a dedicated carnivore was pretty much flashing lights,bells, & whistles, so I ordered it (rare of course). The result was a tender steak, with a good deep flavour & melt in the mouth goodness. It's definately one of the better restaurant steaks I've eaten.
So when I saw it on the slab I had to get a better look, when I saw the price (a ridiculous £8ish a kilo) I had to get out my wallet, I bought the whole thing. I took it with me to my mum & dad's place as a boxing day treat, and had a quick browse of the web for recipes.
Most places on the internet tell you to remove the connecting tissue & then pan fry to no more than medium rare, a bit of onion & garlic, a splash of booze, some salt & fresh pepper, good to go. Unfortunately I was with several less dedicated carnivores than myself, so I cooked it to rare, and put it in foil to rest. I then sliced it across the length to get several discs of steak, reserving the rarest for me I quickly dashed the rest through the pan with some mushrooms & it was time to serve.
When raw, the steak was quite intimidating, shot through with fat & connective tissue. The fast cook rendered the fat & the connective tissue disappeared. Leaving a really tasty tender steak.
If you can find one (and it is one per cow) and the butcher hasn't kept it for him/herself, give it a go, its probably one of the tastier bits of the cow.
(*) That's how I remember it, the actual wording might be different, but was no less forceful.