Combine quinoa, water, and salt in a medium saucepan. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes until water is absorbed and quinoa is tender with visible little tails. Remove from heat, fluff with a fork, and spread on a plate to cool completely - about 15 minutes.

While quinoa cooks, cut broccoli into small bite-sized florets - you want pieces no larger than a nickel so they distribute evenly. Core and dice the apple into 1/2-inch pieces. If you're not assembling immediately, toss the apple with a splash of the dressing to prevent browning.

In a small jar or bowl, combine olive oil, apple cider vinegar, Dijon mustard, honey, garlic, salt, and pepper. Shake or whisk vigorously until emulsified and slightly thickened - about 30 seconds.

In a large bowl, combine cooled quinoa, broccoli florets, diced apple, dried cranberries, and sunflower seeds. Drizzle with about three-quarters of the dressing and toss thoroughly to coat. Taste and add more dressing if needed - the broccoli should be lightly glossy, not swimming.

Gently fold in the crumbled feta. Serve immediately, or refrigerate for up to 2 hours to let flavors meld. If making further ahead, store the dressing separately and toss just before serving.