Drizzle the pork with extra-virgin olive oil and season with salt and pepper. Heat a skillet over medium-high heat and cook the pork in 2 batches, about 2 to 3 minutes per side. Arrange the pork on a platter and cover with foil to keep warm.
Reduce heat under the pan and add the butter to melt. Add the shallots, season with salt and lots of coarse pepper, and cook for 2 to 3 minutes. Sprinkle in the flour and stir 1 minute. Add a generous splash of sherry, and then whisk in the stock and thicken. Stir in the mustard and cornichons.
Serve 3 medallions on each plate with lots of sauce, with bread alongside.