In a bowl, combine cumin, paprika, chili powder, marjoram, garlic, and 2 tablespoons of oil. Add the beef, ensuring it's well-coated with the marinade. For optimal flavour, marinate the beef overnight. If pressed for time, marinate for at least 30 minutes before cooking.
In a heated pan, fry the beef alongside onions on high heat until they develop a nice color. Transfer the beef and onions to a slow cooker. (Alternatively, you can use a pot or continue using the same pan.) Pour in enough stock to cover the meat and season with a teaspoon of salt. For a quicker cook time, set the slow cooker to high and let it cook for 3-4 hours. For overnight cooking, use larger beef pieces and set the cooker on a low setting.
In a separate pan, fry the sweet peppers on medium heat for about 10 minutes. Once done, set them aside.
Once the beef is tender, remove it from the slow cooker. Allow it to rest in its juices for a few minutes before taking it out. Shred the beef using your fingers. In a mixing bowl, combine the shredded beef, fried sweet peppers, chopped parsley, tomato paste, chipotle paste, and a portion of the meat cooking juices.
Lightly brush one side of a tortilla with olive oil. Place it oiled-side-down on a heated frying pan. Sprinkle cheese on one half of the tortilla and top it with the shredded beef mixture. When the underside of the tortilla turns golden brown, fold it over and press gently.
Once cooked, remove the quesadilla from the pan and cut it in half. Serve hot and enhance the flavours with your favourite Mexican toppings like guacamole, roasted tomato salsa, sour cream, jalapenos, salad, and a dash of fresh lime juice.