For
the Sauce:
In a mortar and pestle, smash the garlic and chili into a paste along with the salt. Alternately, you can use a food processor or blender. Add the oil and the scallions, and smash or chop a bit more. Add the cilantro, pulsing the food processor/blender (if you're using it) just a few times.
Transfer the mixture to a jar or bowl, and whisk in the coconut milk, then the lemon juice. Taste, and adjust with more salt or lemon juice if needed. Store in a refrigerator for up to a week.
For the Spring Rolls:
Fill a large shallow dish partway with water. Dip a rice paper wrapper into water just until pliable, about 45 seconds (do not soften completely); allow excess water to drip off.
Place wrapper on a flat surface. Place sauce, cucumber, mango, pepper, avocado and lettuce across bottom third of wrapper. Fold in both ends of wrapper; fold bottom side over filling, then roll up tightly. Place on a serving plate, seam side down. Repeat with remaining ingredients. Serve immediately.