Preheat oven to 375° F (190 degrees C).
In mixing bowl of high speed mixer, beat eggs on high for 5 minutes. Gradually add white sugar in 1/4 C. increments until thick and lemon-colored.
On low speed, add pumpkin and lemon juice.
While eggs are beating, combine flour, cinnamon, baking powder, salt and nutmeg in separate bowl. Fold into the pumpkin mixture.
Line 15x10x1 pan with parchment paper. Spread batter into pan; sprinkle with walnuts.
Bake for 15 minutes or until cake springs back when lightly touched. Check after 12 minutes.
Immediately turn out onto a tea towel dusted with confectioners sugar. Peel paper off and roll cake up in the towel from the short end. Cool on a cooling rack.
While cake is cooling, mix frosting.
When cake is completely cooled, gently unroll. Spread filling over cake to within 1 inch of edges. Roll up again. Cover and chill until serving. Dust with additonal confectioners sugar if desired. Slice and serve.