In a small Mixing Bowl, combine the 1 tbsp sal kosher, 1 tbsp sal kosher, 1 tsp polvo de ajo, 1 tsp polvo de cebolla, 1 tsp pimentón ahumado, and ½ tsp tomillo seco. Mix well. Pat the 4 lb pechuga de res corte plano, capa de grasa recortada a 1/4 inch completely dry with paper towels, then rub the spice mixture all over every surface of the meat. For best results, do this the night before and refrigerate uncovered — but even 30 minutes at room temperature makes a difference.

Preheat your Oven to 325°F. Heat the 2 tbsp aceite vegetal in a large Dutch Oven or heavy oven-safe Pot over medium-high heat until shimmering. Sear the 4 lb pechuga de res corte plano, capa de grasa recortada a 1/4 inch fat-side down first for 4–5 minutes until deep golden brown, then flip and sear the other side for another 3–4 minutes. Sear the edges briefly as well. Remove the 4 lb pechuga de res corte plano, capa de grasa recortada a 1/4 inch and set aside on a plate.

Reduce the heat to medium. Add the sliced 2 cebollas amarillas cortadas en rodajas finas to the same Pot and cook, stirring occasionally, for 8–10 minutes until softened and beginning to turn golden. Scrape up any browned bits from the bottom of the Pot as the 2 cebollas amarillas cortadas en rodajas finas release their moisture.

Add the minced 1 tsp polvo de ajo, 2 tallos de apio picados aproximadamente, and 2 zanahorias picadas aproximadamente. Cook for 2 minutes, stirring frequently. Add the 2 tbsp pasta de tomate and stir it into the vegetables, cooking for another 1–2 minutes until it darkens slightly and becomes fragrant.

Pour in the 1 cup vino tinto seco como Cabernet Sauvignon and let it simmer for 2–3 minutes, scraping up all the browned bits from the bottom of the Pot. This deglazing step is key to a rich, complex braising liquid.

Stir in the 1 lata de tomates triturados, 2 cups caldo de res bajo en sodio, and 1 tbsp salsa Worcestershire. Nestle the 4 lb pechuga de res corte plano, capa de grasa recortada a 1/4 inch back into the Pot fat-side up — the fat will baste the meat as it braises. Tuck in the 2 whole hojas de laurel and ½ tsp tomillo seco around the meat. The liquid should come about halfway up the sides of the 4 lb pechuga de res corte plano, capa de grasa recortada a 1/4 inch; add a splash more 2 cups caldo de res bajo en sodio if needed.

Bring the liquid to a gentle simmer on the Stovetop, then cover the Pot tightly with a lid (or two layers of Aluminum Foil if your lid isn't snug) and transfer to the preheated Oven. Braise for 3 to 3.5 hours, until the 4 lb pechuga de res corte plano, capa de grasa recortada a 1/4 inch is completely fork-tender and a knife slides in with no resistance. Check once at the 2-hour mark to make sure there's still plenty of liquid; add a little 2 cups caldo de res bajo en sodio if it looks dry.

Remove the Pot from the Oven. Carefully transfer the 4 lb pechuga de res corte plano, capa de grasa recortada a 1/4 inch to a Cutting Board and tent loosely with Aluminum Foil. Let it rest for at least 15–20 minutes. Meanwhile, discard the 2 whole hojas de laurel and ½ tsp tomillo seco. Skim any excess fat from the surface of the braising liquid. Taste the sauce and adjust seasoning with 1 tbsp sal kosher and 1 tbsp sal kosher as needed. If you'd like a thicker sauce, simmer it on the Stovetop over medium heat for 5–10 minutes.

Slice the 4 lb pechuga de res corte plano, capa de grasa recortada a 1/4 inch against the grain into 1/4-inch thick slices. This is essential — cutting with the grain will result in tough, chewy meat. Arrange slices on a platter or return them to the Pot with the braising sauce. Spoon plenty of sauce and vegetables over the top, garnish with fresh 2 tbsp perejil fresco de hoja plana picado aproximadamente, and serve.