Preheat oven to 450° with racks in upper and lower third.
Shape dough: Line 2 rimmed baking sheets with parchment paper. Divide dough into 8 equal portions and, if sticky, dust each with flour all over. On a large cutting board, roll each into a 1/2-in.-wide rope, moving your hands from the center outward. Cut in half, lay on lined baking sheets, and let rest in a warm place, covered with a kitchen towel, until doubled, about 20 minutes.
Meanwhile, bring 2 quarts water to a rolling boil, covered, over high heat.
On cutting board, roll each dough rope out again until about 1/2 in. thick and tie into a loose knot with long ends. Twist ends to form pointed “horns.” Leave knots on board as you form them, then carry to stove, along with baking sheets.
Gradually stir baking soda into boiling water (it will foam up). With a slotted spoon, lower 2 knots into water solution and cook 20 seconds (any longer and they’ll taste metallic). Lift out to one of the baking sheets, sprinkle lightly with coarse salt, and re-shape “horns” if they’ve lost their pointiness. Add 1 cup hot water to pot and let return to boil (it will heat up fast). Boil remaining dough knots the same way, adding 1 cup of hot water to pot after each batch.
Bake salted pretzels until deeply browned, switching pan positions halfway through baking, about 15 minutes. Let cool 10 minutes on baking sheets, then loosen.
Meanwhile, blend mustard, sugar, and rosemary in a small bowl. Thin with a little water to give it a dipping consistency.
Serve pretzels warm or room temperature, with mustard.