Mix the fat, sugar, lemon zest and salt until frothy. Separate the eggs and set the egg whites aside. Stir the egg yolk into the fat mass. Mix the flour (except for a tablespoon) with the nuts and baking powder and stir alternately with the milk into the egg mixture. Turn the rhubarb in the reserved tablespoon of flour and fold into the batter.
Line a muffin tin with 2 paper liners each. Spread the batter in it and bake the muffins in a preheated oven at 175 ° C for about 20 minutes.
Beat the egg whites until stiff, finally sprinkling in 100 g of powdered sugar. Take the muffins out of the oven and spread the egg whites on top. Bake for another 10 minutes, until the meringue has got a nice color.