In a medium size bowl, whisk together the ingredients for the marinade and set aside.
½ cup red wine vinegar, 3 Tablespoons spicy brown mustard, 2 Tablespoons Worcestershire sauce, 1 Tablespoon minced garlic, 2 teaspoons dried rosemary leaves, 2 teaspoons onion powder, 1½ teaspoons dried thyme leaves, 1½ teaspoons fine grind sea salt, 1 teaspoon black pepper
Rinse the ribs under cold running water to remove any bone fragments that may be on the ribs. Place the ribs in a freezer bag. I like to place the ribs in a single layer in the bag facing one direction and then place a second layer in the opposite direction so the marinade gets to more of the meat.
3-4 lbs flanken style beef short ribs
Pour the marinade into the bag and squeeze as much air out as you can before sealing the bag. Massage the bag to equally distribute the marinade around the ribs. Place the bag on a tray or in a bowl just in case the bag leaks. Refrigerate in the marinade at least 4 hours, but overnight is best.
Pressure Cooking Flanken Ribs
Remove the ribs from the refrigerator about 30 minutes before you want to cook them. This allows the marinade to warm up some and you will have more liquid for pressure cooking. If you forget to do this, don't worry about it. I have pressure cooked straight out of the refrigerator and it worked fine.
Place the ribs into the inner pot of the Ninja Foodi or whatever pressure cooker you are using. I like to layer them alternating just like I do in the bag for marinating. This way they cook more evenly. Pour in the remaining marinade from the bag. You will have between ¼-½ cup of liquid. If you are using the 8 qt Ninja Foodi, you can add an extra 2 Tablespoons of water if you only have ¼ cup of liquid.
Put the pressure lid on and make sure the valve is in the sealed position. Set the pressure cooker to high pressure and the time to 5 minutes. It will take about 15 minutes for the pot to come to pressure. When the time is up, immediately release the pressure by turning the valve to the vent position. You can also do a natural release for 10 minutes if you prefer, but the ribs are mostly under the liquid so an immediate release does not dry them out.
Open the pressure lid once all the steam has released and the pin has dropped.
Air Frying the Flanken Ribs
You do not have to remove the liquid before air frying. Put the crisping down and select the "air fry" or "air crisp" function, depending on your pressure cooker model. Set the temperature to 400℉/200℃ and the time for 20 minutes. Air fry for 8 minutes. Flip the ribs that are browned on top and move them around so as many ribs are exposed to the heat as possible. Air another 3 minutes and move the ribs around again, flipping them over so both sides brown up. Continue this process until all the ribs are nicely browned on both sides. it can take 15-30 minutes depending on how many ribs you have. Make sure to bring the bottom ribs up to the top so they all brown evenly.
Remove the ribs and place them on a serving platter. You can spoon over some of the juices and serve them as is or you can make gravy with the juices.
Making Gravy
Whisk together the flour and water until you have a smooth consitency. It's important to have all the lumps out.
3 Tablespoons all purpose flour, ¼ cup water, juices and fat from cooking the ribs
Turn the Ninja Foodi or Pressure Cooker on sear/sauté and select medium low heat. Pour the flour and water mixture into the juices in the pot and whisk together. Whisk while heating the mixture and it will start to thicken. Turn the heat to low and cook the gravy for at least 5 minutes to cook out the flour taste. Taste the gravy and season with salt and pepper if needed. If the gravy is too thick, add some water. If it is too thin, mix together 1-2 Tablespoons of flour in ¼ cup of water and repeat the process.
Final Touches
You can serve the gravy over the ribs or on the side. I like to add a sprig of fresh rosemary and some fresh thyme to the platter, but if you don't have any, sprinkle some dried parsley over the ribs to give that pop of green to the dish.
If your pressure cooker doesn't have a crisping lid, remove the ribs after pressure cooking and place them in a single layer onto a baking sheet. Preheat the oven on broil or bake at 500/260℃ and broil/bake the ribs for about 3-5 minutes per side until they are nicely browned. If all your ribs don't fit on the baking sheet, you may have to do this in batches.