Make the marinade: peel and finely chop the garlic cloves. Lightly crush the peppercorns. Mix the garlic, bay leaf, crushed peppercorns, and rosemary into the red wine.
Place the beef steaks in a sealable container with the marinade. Refrigerate for at least 4 hours.
Heat the oil in a pan over medium-high heat. Sear the steaks on both sides until browned, about 2-3 minutes per side.
While the steaks sear, cut the tomatoes and mozzarella into thin slices. Transfer the seared steaks to a wire rack lined with aluminum foil.
Layer the tomato and mozzarella slices over the steaks. Sprinkle with salt and pepper.
Broil at 200°C (400°F) for about 15 minutes until the cheese has melted.
While the steaks broil, strain the remaining marinade to remove the bay leaf and any coarse pepper pieces. In the same searing pan over medium heat, stir the flour into the hot pan drippings for about 1 minute. Slowly pour in the strained marinade while stirring. Bring to a boil.
Remove the sauce from heat and stir in the mascarpone until smooth. If too thick, add milk a little at a time. Season with salt and pepper.