Preheat the grill to high heat. Lightly oil the grill grate.

Grill chicken until juices run clear, about 8 minutes per side. Remove from heat, cool, and slice. Set aside.

Meanwhile, place pecans in a dry skillet over medium-high heat. Cook pecans, stirring frequently, until fragrant, about 8 minutes. Remove from heat and set aside.

To make the dressing: Combine red wine vinegar, sugar, vegetable oil, onion, mustard, salt, and pepper in a blender. Process until smooth.

Arrange lettuce on serving plates. Top with grilled chicken slices, strawberries, and pecans. Drizzle with dressing to serve.