Season the chicken breasts with salt, pepper, and minced garlic on both sides.
Turn on the Instant Pot and select the sauté function. Heat the olive oil in the pot.
Once the oil is hot, add the chicken breasts to the pot and sear them on both sides until golden brown, about 3-4 minutes per side. Remove the chicken from the pot and set aside.
In the same pot, add the mustard, honey, Splenda, and chicken broth. Stir well to combine, scraping up any browned bits from the bottom of the pot.
Return the chicken breasts to the pot, making sure they are coated with the sauce.
Close the Instant Pot lid and set the valve to the sealing position. Select the "manual" or "pressure cook" setting and set the timer for 8 minutes on high pressure.
Once the cooking time is complete, allow the pressure to release naturally for 5 minutes, then carefully do a quick release of any remaining pressure.
Open the Instant Pot lid and check the internal temperature of the chicken with a meat thermometer. It should register at least 165°F (74°C).
Remove the chicken from the pot and transfer it to a serving platter.
Stir the Greek yogurt into the sauce remaining in the pot until well combined. If you prefer a thicker sauce, mix 1 tablespoon of cornstarch with 2 tablespoons of water and add it to the sauce, stirring constantly until thickened.
Pour the sauce over the chicken breasts and serve hot.