Preheat oven to 400 degrees.
In a large nonstick frying pan on medium heat, sauté the sirloin and break it into small pieces. Season with salt and pepper.
When the beef is nearly cooked through, add the onion and cook for 3 minutes, or until softened.
Add the garlic, and stir for 1 minute.
Add in the ketchup and mustard, and stir well.
Divide mixture between flour tortillas, top with cheese and pickles, and roll up tightly on a parchment lined baking sheet.
Bake for 10-12 minutes, or until the tortillas are crispy.