Preheat the oven to 300 degrees.
In a Dutch oven or large, oven-safe pot, heat two tablespoons of olive oil over medium-high heat until shimmering. Pat the belly dry, generously season it with salt and pepper, and allow it to brown on all sides, about 2-3 minutes per side. Once browned, remove the belly and set aside.
Reduce the heat to medium and add the diced onion, making sure to scrape up any browned bits from the beef belly that have adhered to the bottom of the pot. Cook until tender, about 5 minutes, then add the minced garlic and cook for two more minutes.
Add the tomato paste and cook with the garlic and onion until the paste darkens to a terracotta color. Pour in the red wine and beef stock, as well as the thyme and rosemary, and gently stir to combine.
Place the belly in the braising liquid, ensuring that the meat is not fully immersed in the liquid, cover, and put in the oven to braise for 2.5-3 hours, until the meat is fork-tender.
Allow the braised beef belly to chill in the braising liquid overnight in the fridge. This makes sure the meat is as tender as possible and absorbs the most flavor.