Prepare the Starter: In a large mixing bowl, stir the active sourdough starter into the warm water until it dissolves. Add the dark rye flour and mix thoroughly with a large spoon. Cover the bowl with a clean kitchen towel and leave it at room temperature overnight for 8–12 hours. Prepare the Soaker: Place the steel-cut oats and warm water in a small bowl. Stir to combine, cover with a clean kitchen towel, and leave at room temperature overnight for 8–12 hours.
Grease the Loaf Pan: Lightly grease a 5 × 9-inch (roughly 23 × 13 cm) loaf pan with olive oil.
Combine the Dough: Place the prepared starter and soaker in a large mixing bowl. Add the remaining 2 cups dark rye flour, salt, instant yeast, and hulled sunflower seeds. Mix everything together using your hands or a large spoon until evenly combined.
Knead the Dough: Knead the mixture by hand in the bowl or use a stand mixer for a few minutes, working it until it forms a dense ball and is no longer crumbly. The dough will not be elastic like conventional wheat bread dough. Rather than stretching it, knead by pressing and smashing the dough with your hands. Shape the Loaf: Form the dough into a log and place it into the prepared loaf pan. Gently press it down so it settles into the pan. The dough should nearly fill the pan because it will not rise very much.
Rest the Loaf: Cover the shaped dough with a damp kitchen towel and leave it at room temperature for 1 hour.
Preheat the Oven: Preheat the oven to 425°F (220°C).
Bake the Bread: Bake the loaf for 55–60 minutes, until it is dark brown and sounds hollow when tapped with a finger.
Cool Completely: Remove the bread from the loaf pan and transfer it to a cooling rack. Allow it to cool completely to room temperature for at least 2 hours before slicing.