To make dressing, whisk oil with vinegar, dill, shallot, mustard and salt. Set aside.
Fill four 2-cup (500-mL) Mason jars (or 4 tall, narrow re-sealable food containers), by layering ingredients into jars in the following order from bottom to top: dressing, lentils, carrot, feta and spinach. Seal jars and store refrigerated up to 24 hr. Shake well before serving.
Dried French green lentils (Du Puy lentils) are a great substitution for this salad as they hold their shape well. To cook, rinse well and add to boiling water. Reduce heat to medium-low and simmer 25 to 40 min., or until tender but not mushy. 1 cup dried lentils yields approx. 2 cups (500 mL) cooked.