1. Put the parsley, scallion, chives, oil, lemon juice, and mustard into the small bowl of a food processor. Add 1 tablespoon water and 1/4 teaspoon each of the salt and black pepper; pulse until smooth.
2. Heat a grill or grill pan to medium high. Season the salmon with remaining salt and black pepper. Cook 3 to 4 minutes a side for medium, or to desired doneness. Transfer fillets to a plate.
3. Place the snap peas and squash in a steamer basket in a pot of boiling water. Cover and cook until vegetables are tender-firm, about 4 minutes.
4. Spoon 2 tablespoons of the parsley sauce onto each of 4 serving plates. Top each with a salmon fillet and 1 cup of the vegetables.