Adapted from Gourmet Magazine
6 medium (3 lbs.) yellow squash and/or green zucchini
3/4 tsp. salt
1/2 tsp. black pepper
1/4 cup extra-virgin olive oil
2 Tbs. fresh lemon juice
2 tsp. coarse-grain mustard
1/4 tsp. sugar
Prepare grill for indirect-heat cooking over medium-hot charcoal (high heat for gas). Trim the squash and/or zucchini and halve lengthwise, then toss with the salt, pepper and 2 tablespoons of the oil in a large bowl.
Oil the grill rack. Grill vegetables directly over the hottest part of the coals, covered only if using a gas grill, turning over once, until grill marks appear, about 6 minutes total. Move the vegetables to an area of the grill with no coals underneath and grill covered until tender, about 4 minutes more. Transfer to a platter.
While vegetables are grilling, whisk together the lemon juice, mustard, sugar and remaining 2 tablespoons of oil in a small bowl. Pour dressing evenly over the vegetables before serving.