Pour 1 c water into Instant Pot bowl. Place trivet or (Instant Pot safe) steamer basket in bowl.
Place frozen fish filet on trivet. Sprinkle with a pinch of salt and pepper. Place 1 tsp butter on top of fish, and cover fish with 3 slices of lemon.
Replace lid, set vent to venting. Set to MANUAL, high pressure, and adjust time to 8 minutes if less than 1 in thick, or 12 for 1-2 in thick.
Once timer goes off, QUICK RELEASE pressure and remove. Check temperature with instant read thermometer. Temperature in middle of fish should be at least 145 degrees.
Serve with rice or herb couscous.