How to Make It
In a large, shallow dish, coat the flank steak with the garlic and chiles. Drizzle with 2 tablespoons of the olive oil and set aside at room temperature for 2 hours.
In a medium saucepan, bring the chicken stock to a boil over high heat. Gradually whisk in the polenta and bring to a simmer. Cook over moderately low heat, whisking often, until the polenta is thick and no longer gritty, about 45 minutes.
Meanwhile, light a grill. Season the steak with salt and pepper and grill over a hot fire until nicely browned on the outside and medium-rare within, about 4 minutes per side. Transfer the steak to a carving board and let rest for 5 minutes.
When the polenta is tender, stir in the mascarpone, Parmesan and the remaining 2 tablespoons of olive oil. Season the polenta with salt and pepper.
Slice the flank steak against the grain 1/4 inch thick. Drizzle with balsamic vinegar and serve with the polenta.