Preheat oven to 350° F. Lightly grease and flour muffin tins. Spread puff pastry on a lightly floured surface using a rolling pan and cut into 20 strips. Spread a little Dulce de Leche on each strip and place some strawberry slices and chopped walnuts on the edge of the middle of the strip. Carefully fold and roll the strips to form rolls. Arrange in prepared muffin tins; brush with egg.