A simple recursion example.

onion_layers.rb

def peel(vegetable_with_layers)
  if vegetable_with_layers[0] == 'core'
    # base case: have I reached the core?
    vegetable_with_layers[0]
  else
    # not the core... remove one layer
    peel(vegetable_with_layers[0])
  end
end

onion = [[['core']]]
peel(onion)