Season beef on all sides with kosher salt & pepper and cut the beef in half across the grain.
In a medium bowl, combine the rice vinegar, ginger paste, fish sauce, Thai chili sauce, brown sugar, soy sauce, red pepper flakes, mint leaves and water. Mix well.
Place the beef in a gallon-size, resealable bag and pour the marinade over the beef. Seal the bag, squeeze the marinade into the beef (through the bag) and lay the beef flat so the marinade covers it. Place the beef in the fridge 30 minutes, then flip the bag over to ensure all the beef is marinated. Marinate an additional 30 minutes. *See note.
While the beef marinades, chop the onion and garlic and fry the shallots.
After beef has marinated, remove it from the fridge.
Set the Instant Pot to sauté and, after one minute, add 2 tablespoons oil and 1 tablespoon butter. Add the chopped onions and cook until onions are translucent. Add the chopped garlic and cook 30 seconds.
Turn Instant Pot to Keep warm/Cancel, and place the Instant Pot trivet over the onion/garlic mixture.
Set one portion of the beef, fat side up, on the trivet and pour half the marinade over the beef. Set the other half, fat side up, on top of the first half and pour on the rest of the marinade.
Set Instant Pot to Manual and set the timer to 50 minutes. After 20 minutes of natural release, cover the valve with a dishcloth and turn the valve to quick release. When the red button (float valve) on the lid goes down, the lid is safe to open.
Open the lid and transfer the meat to a plate. Remove the trivet.
Use an immersion (hand) blender, to purée the remaining sauce while it is still in the pot. This is the sauce served with the beef. (If you don't own an immersion blender, carefully pour half the liquid to a regular blender and purée it. Repeat with the remaining liquid.)
If serving immediately, pull the beef apart, into chunks, using two forks. If wanting sliced beef, cool the beef about 30 minutes then transfer to the fridge. When the beef is cold, slice it into 1/2 -inch slices.
Serve the beef over a plate of jasmine or white rice and garnish with sliced green onions, fried shallots and small mint leaves. A perfect side is roasted broccoli.
Enjoy!