Cut pork belly lengthwise into 1-inch-thick strips; cut each strip crosswise into ¼-inch-thick slices. Place in a large mixing bowl. Add hạt nêm, black pepper, oyster sauce, MSG, and 1 ½ teaspoons fish sauce; combine thoroughly. Marinate for 10 to 15 minutes.

Meanwhile, cut white parts of scallions lengthwise into strips, and thinly slice green parts of scallions (about 1 cup). Stir together perilla and betel leaves with green parts of scallions in a medium bowl. Set aside.

Heat ¼ cup oil in a medium-size, heavy-bottomed stockpot over medium-high. Once oil just begins to smoke, add marinated pork belly and sliced shallots. Cook, stirring occasionally, until bottom of stockpot begins to develop a small amount of browning, about 10 minutes. Turn off heat; add tomatoes, garlic, and turmeric. Return stockpot to low, and stir gently until browned bits release from bottom of stockpot, about 5 minutes. (If mixture becomes too dry, add a splash of water.)

Once all browned bits are removed from bottom of stockpot, add 2 quarts water. Bring to a boil over high; reduce heat to a bare simmer over medium-low, and simmer for 20 minutes.

While soup simmers, heat remaining 3 tablespoons oil in a large nonstick skillet over medium-high until shimmering. Working in 2 batches, fry tofu on all sides until browned and crispy, 2 to 3 minutes per side. Transfer fried tofu to a paper towel–lined baking sheet.

Stir tofu, mắm tôm (if using), and mẻ into stockpot. Add salt and remaining 3 tablespoons fish sauce.

Add eggplants to stockpot, and bring back to a simmer over medium-high; simmer until eggplants are softened and absorb flavor of broth, about 5 minutes. Add scallion whites, and remove from heat. Stir in reserved scallion greens, perilla leaves, and betel leaves.