For the batter, in a large bowl, beat 1/4 cup melted butter and 3/4 cup sugar until blended. Beat in the egg. In a separate bowl, combine the flour, baking powder, cinnamon, nutmeg and salt. Add about half of the dry mixture to the egg mixture and beat just until combined. Add half of the milk and beat again until combined. Repeat with remaining flour mixture and milk, beating well. Spoon the cake batter over the rhubarb mixture and use an offset spatula or the back of a spoon to spread out evenly.
Rhubarb should be trimmed by about 1" at each end. Trim and discard green leaves. Wash well and slice into 1/2" slices.