To make the puree, heat the olive oil in a pan and add the onion. Cook for 4-5 minutes, add the broad beans and cook for a further 5 minutes. Slow add water a ladleful at a time, bring to the boil and set aside to cool. Puree with a hand blender adding the diced butter as you go. To make the butter sauce, melt one knob of butter in a pan. Add the shallots and sweat them for 2-3 minutes with the peppercorns. Add the wine and reduce by ¾. Add the cream and reduce for a further 2minutes. Remove from the heat and set aside. Put the pan containing the wine reduction back on the heat. Bring back to the boil then slowly whisk in the cold diced butter, once it is all incorporated turn it down, pass through a fine sieve into a warm container and set aside to keep warm. Bring a large pan of salted water to the boil. Add the tagliatelle and cook for a few minutes, drain. Put a knob of butter in a pan; add the broad beans, pasta and a little water. Toss together, then add the pecorino, some pepper, the rocket and 3 or 4 ladlefuls of the butter sauce. Toss in the pasta, serve with the broad bean puree and some fresh rocket.