Bread can be baked 2 days ahead. Store tightly wrapped at room temperature.
1/4 cup millet flour
1/4 cup quinoa flour
1/4 cup amaranth flour
1 cup old-fashioned oats, plus 1 Tbsp for topping
2 1/4 cups buttermilk, divided, plus more for brushing
1 Tbsp. vegetable oil, plus more for pan
4 cups whole wheat flour
2 tablespoons ground flaxseed
2 teaspoons salt
2 teaspoons baking soda
1/4 cup sunflower seeds, plus 1 Tbsp for topping
4 tablespoons unsalted butter, softened
3 tablespoons molasses
1. Mix millet, quinoa, amaranth, 1 cup oats, 1 cup buttermilk, and 1/2 cup water in a small bowl. Allow the sticky mixture to rest 30 minutes.
2. Meanwhile, preheat oven to 350°F. Lightly oil an 8"-diameter cast-iron skillet or cake pan.
In a large mixing bowl, mix whole wheat flour, ground flaxseed, salt, baking soda, and 1/4 cup sunflower seeds, soft butter, molasses, 1 1/4 cups buttermilk, remaining 1 Tbsp. oil AND the prepared oat mixture. Mix with a wooden spoon until dough is smooth, homogenous, and still slightly sticky.
3. Form dough into a ball and place in prepared pan. Brush with buttermilk; top with more oats and sunflower seeds. Cut a large X into the top and bake until golden brown and an instant-read thermometer inserted into the center of loaf registers 190°F, about 1 hour or longer. Let cool in pan.