Begin by preparing the oat and flaxseed mixture, which is essential to the bread's texture and moisture. Combine ⅓ cup (40g) flaxseed and 1⅔ cups (150g) rolled oats in a bowl, then pour 1 cup + 3 tbsps (300ml) of boiling water over them.
Mix well and allow the mixture to sit for approximately 10 minutes, during which time the grains will absorb the water and cool slightly. Once cooled, add the remaining dry ingredients: 3½ cups (450g) bread flour, ¾ cup + 2 tbsps (115g) whole wheat bread flour, ⅓ cup (55g) sunflower seeds, 1 tsp salt, and 1 × ¼-ounce (7g) fast-acting dried yeast.
Gradually incorporate 1⅓–1½ cups (about 350ml) of warm water while mixing until a soft dough forms. Knead the dough for about 5 minutes by hand or with a dough hook until it becomes smooth and cohesive.
Transfer the kneaded dough to an oiled bowl, cover tightly with plastic wrap, and allow it to rise at room temperature for 1–1½ hours until it has roughly doubled in size. Once risen, punch down the dough to release excess gas, then divide it into two equal portions and shape them into rounds.
Place both rounds on a parchment-lined baking sheet, cover the sheet loosely with a large plastic bag, and let the dough proof for approximately 30 minutes until the loaves have doubled in size again.
Preheat your oven to 425°F while the dough proofs. Just before baking, brush the surface of each round with a little milk to create a golden glaze, then sprinkle rolled oats over the top for decoration and texture.
Bake the loaves until they achieve a deep golden-brown color and sound hollow when tapped on the bottom—this typically takes 25–30 minutes, though exact time may vary depending on your oven.