- 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.