Place the meat in a glass dish. Mix together the vinegar, garlic and thyme. Pour over the meat and let marinate for 1 or 2 hours at room temperature. Preheat the oven to 190ºC . Wash the spinach under cold running water; drain and trim off the stems. Spread the leaves evenly over the meat. Lay the carrots, in rows, across the grain of the meat. Place the egg quarters between the rows of carrots. Scatter the onion rings over the eggs and carrots, then sprinkle the parsley, red pepper flakes and salt over all. Carefully roll up the meat with the grain, jellyroll fashion. Secure with toothpicks and lace up with string. Or tie with string at 3 cm intervals. Heat the oil in a dutch oven. Add the meat and brown well on all sides. Add the stock, cover tightly, and bake for 1 hour. Remove the matambre from the pan to a board and let rest for 10 minutes. Using a sharp knife, remove the strings and cut the roll into 1 cm slices. Arrange on a heated platter and moisten with a little of the cooking broth. To serve cold: In Argentina, the matambre is poached, then removed from the pot and pressed under weights until the juices drain off. It is refrigerated until chilled, then served as a hors d'oeuvre, cut into thin slices.