Select a clean 32-ounce wide-mouth glass mason jar to facilitate easy layering and extraction.
Pour 3 tablespoons of the dressing into the bottom of the jar as the foundation layer.
Add the diced cucumbers and red onions directly into the dressing; these hearty vegetables act as a moisture barrier and benefit from marinating.
Layer the chickpeas on top of the cucumbers to provide a sturdy secondary barrier.
Add the cherry tomatoes and cooked quinoa, ensuring the quinoa is completely cooled to avoid generating steam which causes wilting.
Incorporate the feta cheese and sunflower seeds as the penultimate dry layers.
Pack the baby spinach into the remaining headspace, compressing gently to minimize air pockets while ensuring no contact with the liquid dressing.
Affix the lid tightly and refrigerate vertically for up to 5 days.
To serve, invert the jar into a large bowl, allowing the dressing to cascade over the ingredients, then toss to combine.