Set the Instant Pot to sauté. Add 1 Tablespoon of butter until melted. Add the onions and cook, stirring, until softened, 3-5 minutes.
Add the remaining butter and the flour to the Instant Pot and stir continuously until the flour disappears. Turn off the Instant Pot.
Add the garlic powder, broth, heavy cream, Dijon mustard, salt, pepper and nutmeg to the Instant Pot along with the frozen spinach. Stir to combine.
Set to manual high pressure for 5 minutes. When the cook time is done, allow a natural pressure release of 5 minutes. After that time quick release any remaining pressure.
Stir through the shredded parmesan cheese and the hot sauce. Check for salt and pepper and adjust to meet your tastes.
If the sauce is not as thick as you would like, turn off the keep warm button and replace the lid. Allow the dish to sit for 10 minutes or so to thicken.