Chipotle Crema: combine the sour cream, chipotle chilies, lime juice, adobo sauce and salt in the work bowl of food processor fitted with a steel blade. Process until smooth.
Place the sour cream mixture in a mesh strainer and, using the back of a spoon, press the mixture through the strainer into a bowl. Keep stirring until nothing remains but the seeds and pulp from the chilies. Discard the pulp and seeds.
Transfer the crema to a squeeze bottle and refrigerate until ready to use.
Soup: slice the root ends from the leeks, then slice them in half lengthwise and rinse well under running water—spreading out the layers to remove all traces of grit and dirt. Slice the leeks thinly and set aside.
Heat the canola oil in a heavy-bottomed, 6-quart soup pot over medium heat. Add the leeks, zucchini, garlic, serrano chilies and potatoes. Cook—stirring frequently—until the leeks are wilted and transparent, about 10 minutes.
Add the chicken stock, cover and cook for 30 minutes, or until the potatoes are very soft.
Stir in the cilantro and puree the soup in batches in a blender or food processor until smooth, then return to the clean pot.
Add the heavy cream and cook—stirring once or twice—for about 10 minutes to heat the cream through. Season to taste with salt.
To serve, pour the hot soup into bowls or soup plates and drizzle some chipotle crema across the center of each bowl. Nest a small mound of the fried tortilla strips in the center of each.