Make the dough: Combine the flour, salt, pepper, rosemary, and pressed garlic in the bowl of a stand mixer (or a larger mixing bowl). Add the honey to the milk, whisk in the yeast, and let it dissolve. Pour the yeast-milk mixture into the flour mixture and mix until a shaggy dough forms.
Switch to the dough hook and mix at medium speed for 5-6 minutes, until the dough becomes smooth and elastic. If kneading by hand, knead on a lightly floured surface for about 8 minutes. Place the dough in a lightly oiled bowl, cover, and let rise until doubled, about 2 hours.
Make the filling: Heat butter in a skillet and add the sliced onions. Cook until caramelized, about 40 minutes. Add balsamic, salt, and pepper. Let cool.
Assemble and bake: Roll the risen dough into a 12x16 inch rectangle. Spread caramelized onions and cheddar over the dough. Roll into a cylinder. Cut into 12 pieces. Place in a baking dish. Let rise until doubled, about 2 hours.
Preheat the oven to 350. Brush the buns with melted butter and sprinkle with sesame seeds. Bake for 20-25 minutes, until golden brown.
