In the bowl of a standing electric mixer fitted with a dough hook, mix 2 tablespoons of the warm water with the yeast and let stand until foamy. Add the remaining 1 1/3 cups of warm water along with the sugar and swirl to dissolve the sugar. Add the flour and mix at medium-low speed until a firm, pliable dough forms.
Turn the dough out onto a lightly floured work surface and knead for 2 minutes. Roll the dough into a 2-foot-long sausage. Cut the dough into 20 pieces. Cover with plastic and a damp cloth and let rest for 10 minutes.
Roll each piece of dough into a 12-inch-long rope. To form pretzels, shape each rope into a U; cross the two sides of the U over each other, twist and press the ends down on the pretzel. Arrange the pretzels on a lightly floured surface about 1-inch apart and cover with lightly oiled plastic wrap. Let the pretzels rest for 30 minutes.
Preheat the oven to 425°. In a large stockpot, bring the cold water to a boil. Add the baking soda. Carefully slide 5 of the pretzels into the boiling water and boil for 30 seconds, turning once. Using a slotted spoon, transfer the pretzels to a rack to drain right-side up; sprinkle lightly with salt. Repeat with the remaining pretzels, in 3 batches.
Lightly oil 2 baking sheets. Arrange the pretzels on the baking sheets and bake on the upper and middle racks of the oven for about 10 minutes, or until browned all over; shift the pans from top to bottom and back to front halfway through if necessary, for even baking. Let the pretzels cool on the baking sheets for about 5 minutes, then transfer them to a rack. Serve warm or at room temperature, with mustard.
MAKE AHEAD: The pretzels can be frozen for up to 1 month. Thaw, then reheat in a 350° oven before serving.