Grilled Summer Squash and Zucchini

1 pound yellow squash and/or green zucchini 3/4 teaspoon salt 1/2 teaspoon black pepper 1/4 cup extra-virgin olive oil 2 tablespoons fresh lemon juice 2 teaspoons coarse-grain mustard 1/4 teaspoon sugar Prepare grill for indirect-heat cooking over medium-hot charcoal. Read on!