Marinade flank steak: place steak in a large zip-top bag or a large bowl to get it ready for the marinade.
In a large bowl, combine vegetable oil, apple cider vinegar, sugar, minced garlic, Dijon mustard, salt, ground black pepper, diced parsley, and hot sauce. Mix to combine well and pour 2/3rds of the marinade over the steak in the bag or bowl. Reserve 1/3 of the marinade in a small bowl in the fridge to be used later in the sandwich.
Cover the steak and marinade and place in the refrigerator for 4 hours or overnight.
Steak grilling: grill over a preheated charcoal or gas grill or stovetop grill pan until cooked to your desired doneness.
It's hard to say exactly how long in time to grill a flank steak. I typically use a quick-read thermometer and do not cook based on any number of minutes. I like my flank steak medium rare, so I pull it off the grill when the internal temperature is between 120 and 125 degrees F. If you simply must have a time frame, I would start with grilling the steak for 6 minutes per side to get to something close to medium rare.
Allow the steak to rest for 10 to 15 minutes before slicing.
Slice steak against the grain of the meat and store in a sealed container until time to make your sandwich.
Gorgonzola dressing: add the mayonnaise, sour cream, gorgonzola cheese crumbles, and salt and pepper and whisk well to combine.
Taste now to see if you think it needs more salt and pepper. You can stop now (without the buttermilk) if you want more of a mayonnaise-like consistency to spread on sandwiches.
Add buttermilk a little bit at a time and whisk until you reach the right consistency. I usually only need about 3 tablespoons to hit a thick dressing stage but if you're making this for salads, you might want to add more.
Store in a jar or container with a lid in the refrigerator for no more than 2 weeks.
Sandwich assembly: add the orange juice to the remaining 1/3 of the marinade and stir to combine well. This turns the marinade into a citrusy salad dressing. Place a small handful of baby arugula in a small bowl and toss it with a tablespoon of the citrus dressing.
Slice and split your ciabatta and toast it if you desire. Add a tablespoon or so of gorgonzola dressing to the bottom slice of ciabatta.
Top the dressing with a few slices of flank steak and then top the steak with a bit more gorgonzola dressing.
Add pear slices and top the pear slices with the salad dressed arugula.
Close the sandwich with the top bit of ciabatta and serve.