Turn on a multi-functional pressure cooker (such as Instant Pot®) and select Saute function. Cook bacon, turning occasionally, until crisp, 5 to 7 minutes. Remove bacon from pot and drain grease.

Add onion to the pot; saute until browned, about 5 minutes. Remove onion. Saute spinach until wilted, 2 to 3 minutes. Remove spinach and squeeze out excess liquid. Clean out Instant Pot® liner.

Beat eggs in a medium bowl until yellow and frothy. Add onion, spinach, Swiss cheese, half-and-half, salt, garlic powder, lemon pepper. Stir in 1/2 of the cooked bacon.

Spray stackable insert pans with cooking spray; divide egg mixture between the pans. Fill liner with 1/2 cup water. Cover pans with a lid and place inside the liner. Close and lock the lid. Select high pressure according to manufacturer's instructions; set timer for 20 minutes. Allow 10 to 15 minutes for pressure to build.

Release pressure carefully using the quick-release method according to manufacturer's instructions, about 5 minutes. Unlock and remove the lid. Remove casserole from pans and garnish with remaining bacon.