Marinate beef: Combine light soy sauce and calamansi juice in a large mixing bowl. Add beef to the mixture. Cover and let marinate for 30 minutes to 1 hour. Remove beef from marinade and pat dry.

Cook potatoes and carrots: Heat 2 tbsp neutral oil in a deep skillet, sauté pan, or Dutch oven over medium-high heat. Add potatoes and cook, stirring frequently, until browned on all sides. Remove potatoes from the pan and set aside. Repeat step with carrots; remove and set aside with the potatoes.

Sear beef: If pan looks dry, add and heat up another 1 tbsp of neutral oil. Working in batches, add beef in batches and sear until browned on two sides, about 2–3 minutes per side. Set aside.

Cook aromatics: Reduce heat to medium. Add onions and garlic (add more oil if the pan looks dry) and cook until the aromatics are fragrant, 3–4 minutes.

Cook beef: Add seared beef, tomato sauce, and water to the pan. Bring liquid to a boil, then reduce heat to medium-low to maintain a simmer. Dissolve and stir in beef broth cube to incorporate. Cover pan and cook until the beef is fork-tender, about 2 hours, stirring from time to time to make sure nothing sticks to the bottom of the pan. If your mechado looks too dry at any point, add ¼ to ½ cup of water at a time.

Finish vegetables: Once beef is tender and sauce has thickened, add fried potatoes and carrots to the pan. Continue cooking until vegetables are tender, another 10 minutes. Season to taste with patis. If mechado tastes too salty, you can balance it out with 1–2 tbsp sugar. Serve with hot rice.