For the fillet: Preheat the oven to 425 degrees F.
With the tip of a knife, pierce the fillet in eight spots and push a slice of garlic into each incision. Spread the fillet with the mustard, 1 tablespoon of the olive oil, mustard seeds if using and crushed black peppercorns. Sprinkle with a generous pinch of salt.
Heat a large pan over high heat until very hot. Add the remaining 1 tablespoon olive oil and sear the fillet, turning occasionally, until brown all over, 5 minutes. Transfer to an oven-proof dish and roast for a further 15 minutes for medium rare, or until cooked to your liking.
Remove the fillet from the oven, tent with foil and allow to rest for at least 10 minutes before carving--this will ensure the meat is as tender as possible.
For the sauce: After 10 minutes, add the juices from the rested fillet back to the pan on the stovetop. Add the peppercorns, balsamic vinegar, cream and some salt; cook over medium heat until thickened, stirring from time to time, 3 to 5 minutes.
Slice the fillet and serve with the sauce.