Season sirloin with salt, soy sauce and pepper. Set aside.
Heat a pan and drizzle 1 tbsp of oil. Sauté onion for 1 minute. Remove and set aside.
.Heat another tbsp of oil in the same pan and sear sirloin in batches until golden brown. Add garlic, calamansi juice and water and oyster sauce. Cover and simmer for 1 hour. Remove cover and let the sauce reduce.
Transfer into a serving plate, top with sautéed onions and toasted garlic and serve.