Rinse quinoa under cool water in mesh strainer to remove bitterness.
Pour 1 cup water and 1 cup of apple cider into 2-quart pot. Add rinsed quinoa and bring to boil over medium-high heat. Once boiling, reduce heat to medium-low, top pot with lid, and simmer for approximately 15 minutes, or until liquid is absorbed.
Remove quinoa from heat, fluff with fork, and let cool for about 15 minutes.
While quinoa cooling, add remaining ¼ cup apple cider, olive oil, vinegar, mustard, salt and red pepper to large bowl and whisk until combined.
Once quinoa has cooled add to this dressing mixture and stir until combined. Next, stir in onion, Brussels sprouts, dried cranberries, and pepitas to bowl and stir until combined.
May be made ahead and kept chilled overnight. Store leftovers in an airtight container for up to 3 days in fridge.