Season the pork fillet with salt and pepper, and brush with Dijon mustard.
Heat some olive oil in a saucepan.
Brown the pork fillet on all sides, then set it aside on a plate.

Peel the potato and cut it into large cubes.
Peel the onions and cut them into quarters.
In a pot, combine the lentils, potato cubes, onions, sun-dried tomatoes, and thyme.
Add the beef broth, then the whole-grain mustard.
Bring to a boil, then reduce the heat, cover, and simmer for 15 minutes.

Add the pork fillet, cover, and continue cooking for 15 minutes, still on low heat.
Let the meat rest for 5 minutes before slicing it into medallions.
Mix the parsley with the lentils and serve with the pork.