Set the Anova Sous Vide Precision Cooker to 170ºF (76ºC).

Combine pork belly, 2 tablespoons sugar, soy sauce, mirin, and 1 teaspoon fish sauce in a large zipper lock or vacuum seal bag. Seal the bag using the water immersion technique or a vacuum sealer on the moist setting.

Place the bag in the water bath and set the timer for 10 hours. Cover the water bath with plastic wrap to minimize water evaporation. Add water intermittently to keep the pork belly submerged.

When the timer goes off, remove the bag from the water bath. Remove the pork belly from the bag and pat dry with paper towels. Slice into ¼-inch thick pieces.

Heat oil in a large skillet over medium-high heat. When the oil begins to shimmer, add the pork belly slices and sear until golden brown on each side, 1 to 2 minutes total.

Meanwhile, prepare the ramen noodles according to package directions. When the noodles are soft, add the lime juice, remaining 4 tablespoons sugar, and 4 teaspoons fish sauce.

Divide ramen noodles and broth between 2 serving bowls. Top with pork belly slices, bean sprouts, and cilantro. Serve.