Cotija is a crumbly, salty cheese sold in Latin markets. Ricotta salata -- available at Italian and gourmet markets -- is a good substitute. You can make the salad and vinaigrette one day ahead; refrigerate them separately.
Put watercress, yellow squash, radishes, and onion in a large serving bowl.
Whisk together vinegar, mustard, salt, and honey in a small bowl. Whisking constantly, pour in the oil in a slow, steady stream; whisk until emulsified. Dress salad; sprinkle with the cheese.