Put the cucumber, chili pepper, red pepper flakes, 1/4 cup of the peanuts, 2 tablespoons of the coconut, lime juice, sugar and 1 teaspoon salt in a large bowl. Toss gently.
Heat the oil in a small skillet over medium-high heat. Add the mustard seeds. When they start to sizzle and pop, stir in the cumin and remaining 2 tablespoons coconut. Turn off the heat.
Pour the mustard seed mixture over the cucumbers and toss everything together (it will sizzle). Taste to make sure the salad has a good balance of salty-sweet-hot adding more salt, sugar, chili or citrus to taste.
Serve in bowls sprinkled with the remaining peanuts and cilantro or mint.
Mix the salad shortly before serving so it doesn't become watery.
If you can't find brown mustard seeds, simply omit them.