Prepare the dressing by whisking together the olive oil, balsamic vinegar or lemon juice, Dijon mustard, salt, and pepper in a small bowl. Adjust seasoning to taste.
Assemble your Mason jar salads in the following order:
Divide the dressing evenly among the Mason jars, pouring it into the bottom of each jar.
Layer the cooked quinoa or farro on top of the dressing in each jar.
Add the cherry tomatoes, cucumber, bell peppers, shredded carrots, and any additional vegetables you're using, dividing them equally among the jars.
Layer the diced grilled chicken breast on top of the vegetables in each jar.
If using, sprinkle the crumbled feta cheese and sliced almonds or seeds over the chicken in each jar.
Top each jar with the mixed salad greens, pressing them down gently to fit.
Seal the Mason jars tightly with their lids and store them in the refrigerator until ready to eat. The salads can be stored for up to 3-4 days.
When ready to enjoy, simply shake the Mason jar to distribute the dressing evenly, then pour the salad into a bowl or eat directly from the jar.