Slice each chicken breast horizontally to make 2 thinner cutlets (4 pieces total), then season both sides with salt and pepper.
Set the Instant Pot to Sauté and heat the butter and olive oil until the butter melts.
Add the minced garlic and chopped onion and cook, stirring, for about 2 minutes until fragrant and softened, then turn the Instant Pot off.
Add the seasoned chicken pieces to the pot, then pour in the chicken broth, lemon juice, lemon zest, Italian seasoning, crushed red pepper flakes, and the additional 1/4 teaspoon salt.
Seal the lid and set the valve to sealing. Pressure cook on HIGH for 8 minutes, then perform a quick release when the cook time ends.
Carefully remove the lid and transfer the chicken to a plate; set aside.
Turn the Instant Pot to Sauté and simmer the cooking liquid for about 5 minutes to reduce slightly.
Stir in the heavy cream.
Whisk the cornstarch with 1 1/2 teaspoons water to make a slurry, then stir it into the simmering sauce and cook for about 2 minutes until the sauce thickens.
Turn the Instant Pot off and stir in the shredded Parmesan until melted and smooth.
Spoon the creamy lemon sauce over the reserved chicken and serve.
