Sprinkle the steak with salt and pepper.
Using a grill pan, cook the steak until it's medium to medium-well. Place the steak on a plate to cool, and the cut into slices or chunks.
Put the steak slices in the refrigerator to cool completely. It's helpful to cook the steak earlier in the day or the day before.
Also, if you haven't made the homemade balsamic vinaigrette, do so now. Store any leftover dressing in the refrigerator for up to a week.
Put the steak salads together. Divide all of the ingredients among the mason jars.
Start with the balsamic dressing and continue with the red onion, red peppers, cucumbers, steak, crumbled blue cheese, chopped walnuts, dried cranberries, and end with the mesclun greens.
Put the lid on tightly. Store in the refrigerator for up to 4 days.
When you're ready to eat, dump the salad into a large bowl and enjoy!