Halve the red onion lengthwise and thinly slice it into half-moons. Seed the tomatoes and dice them into 1/2-inch cubes—this removes excess moisture so your salad won't get watery. Drain and rinse the butter beans thoroughly under cold water. Remove the stems from the parsley and finely chop the leaves. Have all ingredients prepped and ready before you begin assembling the salad.

Place the sliced red onions in a large bowl and sprinkle with sumac and 1/2 teaspoon of salt. Using your fingers or the back of a spoon, gently rub and massage the mixture into the onions for 30-60 seconds. This breaks down the onion's cell walls slightly, making them softer and more pliable while allowing the sumac to coat each slice evenly and add its bright, tangy flavor. The massage also begins mellowing the raw onion's sharpness.

Add the diced tomatoes, drained butter beans, and chopped parsley to the bowl with the massaged onions. Gently fold these ingredients together to distribute them evenly. I like to fold rather than stir at this point because it helps the delicate tomato pieces maintain their shape and prevents them from breaking down too much before the dressing is added.

In a small bowl or measuring cup, whisk together the olive oil, red wine vinegar, and lemon juice until emulsified. Pour this dressing over the vegetables and beans, then add the remaining 1/2 teaspoon of salt, the Aleppo chili flakes, and black pepper to taste. Gently toss everything together until well combined, making sure the dressing coats all the ingredients evenly. I find adding the acidic elements (vinegar and lemon juice) with the oil helps the flavors distribute more evenly than adding them separately.

Let the salad sit at room temperature for 20-30 minutes before serving. This resting time allows the flavors to meld and deepen, the beans to absorb some of the dressing, and the onions to continue softening. The salad actually tastes better the longer it sits, so don't hesitate to make it a few hours ahead if you prefer.