Soak the oats. Place the steel-cut oats in a large bowl and cover generously with water. Let soak for about 6 hours.
Drain well. Pour the oats into a fine-mesh strainer and let as much soaking water as possible drain away. Do not rinse unless necessary.
Combine. Transfer the drained oats to a large bowl. Add the sunflower seeds, pumpkin seeds, ground flaxseed, cooked quinoa, psyllium husk and salt. Stir well.
Add the liquid. Mix 1¼ cups water with the apple cider vinegar. Pour it over the oat mixture and stir very thoroughly. Add the remaining water gradually only if needed; the mixture should be very moist and sticky but should not swim in liquid.
Rest. Let the mixture stand for 10–15 minutes, until the psyllium and flax absorb moisture and the mixture thickens enough to hold its shape.
Fill the pan. Heat the oven to 350°F (175°C). Line a standard 9 × 5-inch loaf pan with parchment. Transfer the mixture to the pan and press it down very firmly, especially into the corners. Smooth and compress the top.
Bake. Bake for 60–70 minutes. Carefully lift the loaf from the pan and remove the parchment, then place it directly on the oven rack. Bake about 10 minutes more, until the exterior feels firm and crusty.
Cool completely. Set the loaf on a rack and let it cool completely before slicing. The oats, flax and psyllium continue setting as the loaf cools.
A friendly note: These recipes are offered for general wellness inspiration. Individual nutrition needs and food restrictions vary; ask your healthcare professional for personalized guidance.