Stir milk, water, oats, bananas, cinnamon, allspice, nutmeg, and a big of salt together in the inner pot of a pressure cooker.
Lock lid in place and turn steam vent handle to Sealing. Cook on High Pressure for 12 minutes for firmer steel cut oats, or 14 minutes for softer oats.
Turn steam vent handle to Venting to quick-release pressure. Remove lid and stir butter and brown sugar into oatmeal. Garnish with walnuts.