PreparationDressing: Pour buttermilk into a measuring glass or small bowl. Heat oil in a small saucepan or skillet over medium-high until shimmering. Add chiles, curry leaves, cumin seeds, and brown mustard seeds one at a time, pausing for a second between each addition and swirling pan. Cook until spices are toasted and curry leaves sizzle and pop, about 10 seconds (take care as oil will splatter!). Quickly pour spice oil into buttermilk; stir to combine. Taste chaas dressing and season with salt. Do Ahead: Dressing can be made 1 day ahead. Cover and chill. Salad and assembly: Soak radishes in a bowl of ice water until they curl, about 10 minutes. Drain and pat dry. Toss half of the radishes in a large bowl with lettuce, cucumber, peas, mint, and cilantro. Drizzle in dressing; toss to coat. Taste salad and season with salt if needed. Top with remaining radishes.