Heat the oil in a large pan and cook the onion over medium head for 10 minutes or until very soft and golden brown. Add the garlic, cumin, cinnamon & chilli and cook, stirring for another minute.
Add the steak and turn to coat with the spices, then stir in the stock, scraping the bottom of the pan. Bring just to the boil, reduce the heat and gently simmer, covered, for 1 hour. Uncover and cook for a further 1-1 1/2 hours, or until the liquid has just about evaporated. Stir frequently towards the end of the cooking time to prevent burning on the bottom of the pan.
Shred the meat, using two forks to pull it apart, then leave to cool.
Preheat the oven to warm 160 degrees celcius/ 315 degrees farenheight. Wrap the tortillas in foil and place in the oven for 10 minutes to soften. Remove from the oven and leave wrapped for another 5 minutes. Working with one at a time (and leaving the other tortillas wrapped as you work), place a quarter of the meat on one tortilla and fold in the sides to enclose. Set aside, fold-side-down while you fill the remaining tortillas.
Heat about 2cm of oil in a frying pan to moderately hot. Place a few chimichangas fold-side-down in the pan, then turn and cook the other side until golden and crisp. Repeat with the rest of the chimichangas.
Drain on a paper towel and serve immediately.
Serve with Mexican Rice!