Begin by gently heating the milk, butter, and molasses together until the butter has fully melted, infusing the liquid with a rich, sweet aroma. Then, add the cold water and egg to this mixture, blending well to combine. In a large bowl, whisk together the bread flour, salt, and yeast until evenly mixed. Pour in the milk and water mixture and stir thoroughly with a wooden spoon until a rough dough forms. There's no need for vigorous kneading here; a gentle mix suffices. Cover the bowl with a damp towel and let the dough rest overnight in the refrigerator, allowing it to develop flavor and structure slowly. The following morning, generously flour your work surface and knead the dough briefly to a smooth consistency. Divide it into eight equal portions, shaping each into a neat ball. Place these dough balls onto a baking sheet lined with parchment paper, cover again with a damp towel, and let them rise for about an hour or until doubled in size, creating soft and airy buns. Preheat your oven to 220 degrees Celsius (428 degrees Fahrenheit). Before baking, brush the buns lightly with milk, heavy cream, or an egg wash for a beautiful golden finish. Optionally, sprinkle sesame seeds on top for an added nutty flavor and texture. Bake the buns for 15-20 minutes until they turn a lovely golden brown. Allow them to cool to avoid sogginess when assembling. For the toppings, finely chop the onions and sauté them in a little butter over medium heat until they turn a deep golden color and caramelize; this process takes around 10-15 minutes and enhances their natural sweetness—remember to stir occasionally to prevent burning. Rub the sirloin steaks lightly with crushed garlic and season with salt and pepper. Grill or pan-fry the steaks, bacon strips, and eggs to your preferred doneness. Toast the brioche buns by halving them and lightly browning the cut sides. To assemble the Chivito, spread a thin layer of mustard on the bottom half of each bun. Layer with a sirloin steak, followed by a slice of ham, mozzarella, crispy bacon, a slice of fresh tomato, and some crisp lettuce. Top each with a perfectly fried egg. Just before serving, sprinkle generously with flavorful chimichurri sauce and a few caramelized onions for an irresistible finish. Enjoy immediately for the best taste and texture.