5.3 Les Prolongations, 76, blvd de Grenelle in the 15th , 01.45.75.62.05, (Metro: Dupleix) is the second resto of Nathalie et Jean of the Dernier Metro 45 meters away and resembles it in many ways although to me it seemed even more basque but with slighly fewer menu items. Funnily enough, I ordered practically the same things I did at the Dernier which either shows a limited horizon or steadfast consistency.
My friend J had the vegetable soup which sure looked watery and he said it was; our friend R. had the pigs' ears which I thought were the tops but he wished they had a spritz of lime juice as he's used to; and I had the duck hearts, which when "adjusted" with some of the house herbed espilette, salt and pepper mix, were just fine.
Then they both had the tripes Basque-style which they loved and I equally appreciated my axoa with mashies, which they thought had a bit too thick a sauce but I did not.
To finish, R. ordered the Basque cheese platter which he shared and was very good; R. had a gateau Basque which was very dry and ? old; and I had a fine creme caramel that escaped my camera but was quite good.
Go back? I think I'd choose the Mother House if I do. A tad better. If you go, ask for the round table in an almost private room in the back by the toilet.