Set the Instant Pot to the sauté setting. Add the ground sausage and cook, breaking it up as it browns. Scrape up any bits from the bottom of the pot.
In a large mixing bowl, crack the eggs and add the sea salt. Whisk until the yolks are fully incorporated and the mixture is light and airy. Add the whole milk and whisk again.
Once the sausage is browned, add the chopped mushrooms and diced bell peppers. Cook for 3-4 minutes until the mushrooms release their liquid and most of it evaporates.
Turn off the Instant Pot and add the baby spinach. Stir continuously until the spinach is wilted.
Grease a 7-inch pan with nonstick cooking spray. Add the sausage and vegetable mixture to the pan. Pour the egg mixture over the top and gently mix with a fork to ensure the eggs reach the bottom.
Sprinkle the crumbled feta cheese over the top. Cover the pan with aluminum foil.
Add 1 ½ cups of water to the Instant Pot and place a 3-inch trivet inside. Place the covered pan on top of the trivet and lock the lid. Set to cook on high pressure for 45 minutes.
Once the cooking time is complete, perform a quick release of the pressure. Carefully remove the lid and the aluminum foil. Let the frittata cool for a few minutes before slicing and serving warm.