In small bowl, combine onion, vinegar, red pepper flakes, maple syrup, sumac, salt, and pepper, and set aside.
In large bowl, add 1 Tbsp olive oil and kale, and massage kale for a few minutes with your hands. Let stand for 20 minutes.
To assemble salad, toss kale, pickled onion dressing, and remaining olive oil with cabbage, mint, dates, and farro. Keep cool or refrigerate for up to 4 hours before enjoying.