Step 1In a deep, heavy pot or automatic electric deep-fryer, heat approximately 4 inches of oil to a temperature of 375 degrees. Fry the shells, gently curving them to form a hard taco shape (dip one half of the shell in the oil to firm using one set of tongs, then slowly add the remaining half using a second set of tongs, curving the shell as it fries to form the shape). Remove from the oil, carefully shaking off excess oil, and transfer to paper towels to drain.
Step 2Heat a large nonreactive sauté pan over medium-high heat. Add the oil and swirl it to coat the bottom of the pan. Evenly distribute the pastrami slices and cook them, turning occasionally with tongs, until their fat begins to render and the meat begins to brown slightly, about 2 minutes. Then push the meat to one side of the pan and spread the sauerkraut in the other side, stirring it occasionally until heated through, about 2 minutes longer. Remove from heat.
Step 3With the back of a teaspoon, smear the mustard inside the taco shells, taking care not to break them.
Step 4Evenly distribute the sauerkraut among the taco shells. Evenly spread the pastrami on top. Garnish with 3 pickled jalapeño slices per taco, and top each filling with a dill sprig. Serve immediately.