Place Onion in bowl of Food Processor and chop well. Remove Onions and place in a paper towel. Squeeze out all the liquid. Place Onions back into Food Processor. Add Garlic, Marjoram, Rosemary, Oregano, Salt and Pepper. Process, until Garlic is minced.
Add Ground Lamb and/or Beef and Process until well combined.
Press Meat mixture into Loaf Pan, until very tight and compact. Tightly cover with tin foil. Poke a few vent holes into tin foil.
Add 1.5 cups of water to Pressure Cooker cooking pot and add Trivet. Place Loaf Pan on Trivet.
Lock on Lid and close Pressure Valve. Cook at High Pressure for 15 minutes. When Beep sounds, allow a full Natural Pressure Release.
Remove Loaf Pan from Pressure Cooker and let rest for 15 minutes. Remove Gyro Meat from Loaf Pan and slice thin. Crisp under Broiler or in Air Fryer, if desired.