Using highest sauté function, cook pancetta in Instant Pot, stirring often, until browned and fat is well rendered, 6 to 10 minutes. Using slotted spoon, transfer pancetta to paper towel–lined plate; set aside for serving.
Add onion and salt to fat left in pot and cook, using highest sauté function, until onion is softened, about 5 minutes. Stir in garlic, anchovies, fennel seeds, and pepper flakes and cook until fragrant, about 1 minute. Stir in tomatoes and their juice, broth, and water, scraping up any browned bits, then stir in pasta.
Lock lid in place and close pressure release valve. Select Pressure Cook function on High and cook for 5 minutes. Turn off Instant Pot and quick-release pressure. Carefully remove lid, allowing steam to escape away from you.
Stir in Pecorino and season with salt and pepper to taste. Transfer to serving dish and let sit until sauce thickens slightly, about 5 minutes. Sprinkle with parsley and reserved pancetta. Serve, passing extra Pecorino separately.
Notes
*Pressure Cook and Manual buttons are interchangeable