Rinse the quinoa and cook it in plenty of water with a pinch of salt for about fifteen minutes until tender. Drain and let steam for a moment with the lid on.
Slice the fennel into thin wedges. Heat half of the olive oil in a frying pan and sauté the fennel with the chopped garlic over medium heat until soft and lightly coloured, about eight minutes.
Rub the pork tenderloin with salt, pepper and mustard. Heat the remaining oil in a separate pan and brown the meat on all sides, then cook for a few more minutes on lower heat until it's pink inside. Let the pork tenderloin rest briefly and slice. Serve with the fennel, the quinoa and a generous handful of chopped parsley.