Sauté the onion in the oil.
Add the half and half and chicken stock.
Bring to a boil and add roux.
Whisk over medium heat until thickened and smooth.
Add the cucumber, lemon and parsley and return to a boil.
Sprinkle both sides of the Mahi-Mahi and dust with flour.
Sauté in butter to a golden brown and top with sauce.