Every time I make soda bread, I wonder why I don't make it more often! It is so easy, and I love the distinctive taste. This version is full of hearty rye flavor, and tastes delicious toasted and topped with salted brown butter.
Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.
Mix sunflower seeds, oats, 1/2 teaspoon caraway seeds, and poppy seeds together in a small bowl. Set aside.
Combine remaining 2 teaspoons caraway seeds, rye flour, all-purpose flour, baking soda, and sea salt in a large bowl. Make a well in the center. Pour in 1 3/4 cups buttermilk and stir with a wooden spoon until dough starts coming together. Knead briefly in the bowl until no dry spots remain.
Transfer dough to the prepared baking sheet and shape into a round loaf. Use damp hands if dough is sticking as you shape it. Cut a deep cross into the top using a sharp knife. Brush with 1 tablespoon buttermilk and sprinkle seed-oat mixture all over the top and into the cracks.
Bake in the preheated oven until top and bottom are golden brown and a toothpick inserted into the center comes out clean, 35 to 40 minutes. Cool completely before slicing.