Set Instant Pot to sauté mode and add olive oil. Cover tenderloins with the dry rub. When oil is hot, set seasoned tenderloins in the pot and allow to brown by searing 3-5 minutes on each side. Remove from Instant Pot and set aside.
Add onion slices to Instant Pot. Stir to coat in any remaining oil and as onions release liquid, scrape bottom of pot to release anything that may have stuck. Allow to cook for a few minutes until they start to brown. Add garlic, stir, and allow to cook for another minute.
Add chicken broth, Dijon mustard, and apple cider vinegar to the pot and stir to combine. Add seared tenderloins as well as any liquid that may have accumulated.
Secure and lock lid, then set to high pressure for 3 minutes. It will take a bit of time to get to pressure, but once the three minute timer is done, allow pressure to naturally release for 15 minutes.
Transfer tenderloin to a cutting board and loosely tent with foil. Pour everything else to a bowl or measuring glass and set aside.
Set pot back to sauté mode and add butter. Once melted, add flour and continue to whisk while it cooks to create a roux, about 5 minutes. Add all of the liquid with the onions back to the pot and stir well. It should thicken within a minute or so.
To serve, slice tenderloin and serve with gravy.