Hydrate multigrain mix. In a large mixing bowl, combine the rolled oats, flaxseeds, sunflower seeds, and millet with one tablespoon of warm water. Let it sit for 10 minutes to soften.
Mix dough. Add the sourdough starter, bread flour, water, sugar, and the hydrated multigrain mix. Stir until incorporated. The dough will look slightly sticky but should be workable.
Add salt and yeast. Sprinkle the salt and yeast over the dough. Knead until smooth and elastic, about 8–10 minutes by hand or 5 minutes in a stand mixer. If using a bread machine, select the dough cycle and let it handle the kneading. The dough should be slightly sticky but not too hard to work with. If needed, add a small amount of extra water or flour.
First rise. Cover the dough and let it rise in a warm place for 60–90 minutes or until nearly doubled in size.
Shape the rolls. Turn the dough out onto a lightly floured surface. Divide into eight equal pieces using a sharp knife. Shape each piece into a round roll by pinching the bottoms to smooth the tops.
Add seed topping. Brush the tops of the rolls lightly with water or egg wash. Sprinkle with the sesame, poppy, and sunflower seed mixture, pressing gently to help them stick. Place the rolls on a parchment-lined baking sheet dusted with cornmeal.
Second rise. Cover the rolls with greased plastic wrap and let them rise for 45–60 minutes until puffy.
Bake with steam. Preheat your oven to 450°F with one rack in the center and another in the lower position. Place an empty metal baking sheet on the lower rack. Once the oven is preheated, pour 1 cup of water onto the hot baking sheet to create steam. Bake the rolls on the center rack for 17–21 minutes, until golden brown.
Cool. Transfer the rolls to a wire rack to cool for at least 30 minutes before serving.