Dilly Rolls

Gently heat milk until warm to the touch (an instant-read thermometer should register 120°F–130°F). Add sugar and yeast and whisk to combine. Add buttermilk; let sit until yeast starts to foam, 5–10 minutes.
Combine onion, fresh dill, dill seeds, salt, and 2 1/2 cups flour in a large bowl. Stir in milk mixture and 2 tablespoons butter, adding more flour as needed (dough will initially be fairly wet), until a sticky dough forms. Turn out onto a lightly floured surface and knead until dough is smooth and elastic, about 5 minutes.
Transfer dough to a lightly buttered bowl and cover with plastic wrap. Let rise in a warm, draft-free area until doubled in size, about 1 hour. Punch down dough and let rise until doubled again, about 1 hour. Punch down dough and, using a pastry scraper or a sharp knife, divide into 12 pieces.
Line a baking sheet with parchment paper and lightly butter top. Cupping your hand over and using your palm, roll each piece of dough on work surface into a ball and place on prepared baking sheet, spacing at least 2" apart (you need to leave enough room so that they're not touching after they've risen). Brush tops with remaining 2 tablespoons butter and cover loosely with plastic wrap; let rise until almost doubled in size, 20–30 minutes.
Meanwhile, preheat oven to 400°F.
Bake rolls until puffed and deep golden brown on the tops and bottoms, 15–20 minutes. Transfer to a wire rack and let cool slightly before serving.
DO AHEAD: Rolls can be made 3 hours ahead. Reheat before serving, if desired.