Soak the steel-cut oats: Place the steel-cut oats in a bowl and cover generously with water. Soak for 6 to 7 hours, until the oats are pliable and can be pressed between your fingers but still retain their shape. Do not oversoak. Drain very thoroughly using a fine mesh sieve.

Combine the dry ingredients: Add the drained steel-cut oats, flaxseeds, sunflower seeds, pumpkin seeds, quinoa flakes, psyllium husk, and salt, if using, to a large bowl. Mix until evenly combined.

Mix the liquids: In a separate container, stir together the water and apple cider vinegar.

Mix wet with dry: Pour over the oat and seed mixture. Mix thoroughly. Stir very well, making sure the psyllium and liquid are evenly distributed and there are no dry pockets.

Rest: Let the mixture sit for 25 to 30 minutes. This gives the psyllium time to fully hydrate and helps bind the loaf.

Pack the loaf pan: Transfer the mixture to a nontoxic nonstick or parchment-lined 8 ½ x 4 ½-inch loaf pan. Press the mixture down very firmly and evenly. Compacting the mixture well is important for a loaf that holds together when sliced.

Bake: Bake on the middle rack at 325°F (163°C) for 90 minutes. Optional for a firmer crust: Carefully remove the bread from the pan for the final 10 to 15 minutes of baking and place it directly on the oven rack or a baking sheet.

Cool: Let the porridge bread cool completely, preferably on a cooling rack, before slicing. Allow at least 30 minutes so the loaf has time to firm up.