Season both sides of the flank steak liberally with salt and black pepper.

Set a large cast iron (or stovetop grill pan) over medium-high heat with 2 tablespoons olive oil and butter. Once hot, sear the steak, untouched, for 4 to 5 minutes on the first side. Flip over and cook for another 2 to 3 minutes on the second side for about medium rare. Transfer to a plate and let rest for about 5 to 8 minutes before slicing.

Lay the bread slices on a baking sheet in a single layer and drizzle with remaining 2 tablespoons olive oil. Toast under the broiler on both sides.

To assemble the sandwiches, spread half of the bread slices with grainy mustard and the other half with bacon jam (or tomato jam if using). Lay a piece of lettuce on top of the jam halves of bread and top each with 2 slices of Brie. Divide the sliced steak amongst each sandwich and then sandwich together with the mustard halves. Skewer with a long toothpick to hold in place and serve. Enjoy!