Grilled Watermelon Pizza

1 cup sliced red onion
3 tablespoons red wine vinegar
1 tablespoon sugar
1/4 teaspoon kosher salt
2 teaspoons extra-virgin olive oil
1 (3/4-inch-thick) slice seedless watermelon (from a 9-inch-diameter watermelon)
1 ounce creamy blue cheese, crumbled (about 1/4 cup)
2 tablespoons coarsely chopped pecans, toasted
2 tablespoons small basil leaves

Heat a grill or grill pan to medium-high heat.
Combine onion, vinegar, sugar, and salt in a medium bowl;
let stand 15 minutes or until onion is softened, tossing occasionally.
Combine 2 teaspoons of the onion liquid and 2 teaspoons olive oil, stirring with a whisk.
Drain onion, and discard remaining liquid.
Brush olive oil mixture onto both sides of watermelon slice.
Grill watermelon 3 to 4 minutes on each side, until well marked and juicy;
place on a serving platter or cutting board.
Top watermelon evenly with pickled onions, blue cheese, and pecans.
Sprinkle with basil.
Cut into 8 wedges.