In a large sauté pan over medium-high heat, add the olive oil. Once heated, brown each side of the pork belly for 2-3 minutes until the outside is no longer pink.
Reduce the heat to medium. Add the onion, garlic, and tomato. Stir to combine with the pork belly and cook just until the onion softens.
Add the shrimp paste and red chili flakes to the pan. Stir to combine. Once mixed together, add the water and vinegar. Cover and simmer over medium heat for 10 minutes.
Remove the cover and continue to cook uncovered for another 1-2 minutes until the sauce slightly thickens. Season with additional red chili flakes if desired. Garnish with green onions and a pinch of sugar if using. Enjoy!