Prepare marinade for pork belly. Season the pork belly with salt and pepper. Combine minced garlic with juice of ½ lemon, 1 tablespoon dijon mustard, ½ cup water, ¼ cup olive oil, 1 teaspoon red pepper flakes, an 1 teaspoon Italian seasoning, stirring to combine. Pour marinade over pork belly and marinade for at least 1 hour or overnight. Let come to room temperature for 30 minutes before braising.
Place pork skin side facing up in the instant pot along with the marinade and ½ cup white wine. Set options to pressure cook on high for 1 hour. Release steam when 1 hour has ended. Pork belly should be fork tender and easily tear apart.
Remove pork belly from the instant pot. Heat medium non-stick pan over medium-high heat. Add 1 tablespoon oil and carefully lay pork belly, fat side down on the pan. Sear until browned and caramelized about 3-5 minutes. Remove from pan and let rest for 10 minutes.
In the same pan, add shallot and garlic and saute for 30 seconds or until lightly browned. Add ½ cup of the braising liquid, ½ tablespoon mustard and 2 sage leaves. Bring to a boil, reduce heat to medium-low and simmer until slightly thickened, about 3 minutes. Add butter and season with salt and pepper. Slice pork belly into slices and serve with shallot mustard sauce.