Trim any connective tissue or fat from your steak. Skirt steaks have one side that has connective tissue. Just grab it with a paper towel and pull it off.
Season both sides with salt and pepper and drizzle with oil; rub in on all sides. Grill over medium high for about 6 minutes per side for flank steak or 4 minutes per side for skirt steak.
Rest the steak covered in foil for 10 minutes before slicing across the grain.
While the steak rests, assemble the salad. Add the greens, blue cheese, onions, tomatoes and strawberries to a bowl and toss to combine. Set aside.
In a small bowl add the balsamic vinegar and mustard and whisk together with a fork or whisk. Slowly drizzle in the oil in a steady stream while whisking. Add salt and pepper and whisk together well.
To serve, either add all the steak to the salad, add dressing and toss or make individual plates and dress each one individually.