Preheat oven to 400°F (200°C). Spread Gwillimdale Farms beets out on a parchment lined baking sheet and cook for 40 minutes or until tender.
For the dressing: In a small bowl, whisk together balsamic vinegar, canola oil, honey, dry mustard, and salt.
Once the beets are cooked to fork-tender and cooled for 20 minutes, mix them in a medium-sized bowl along with the spinach and feta cheese.
Layer salad in four jars, each one as follows: ½ cup spinach, ½ cup beet mixture, ½ cup spinach, ½ cup beet mixture. Top with 1 tbsp (15 mL) of balsamic vinegar dressing and 2 tsp (10 mL) walnuts.