1. Put purslane, butter lettuce, frisée, cucumber, and mint in a large serving bowl.
2. Sprinkle garlic with salt, then mash into a paste with the flat side of a chef's knife. Scrape paste into a bowl and whisk with oil, yogurt, and chile flakes.
3. Pour dressing over greens and toss gently but thoroughly.
* Peppermint is much stronger than the common grocery-store spearmint, with darker, more elongated leaves and purplish stems. Find it at well-stocked grocery stores (or grow your own).