03Heat grill to high. Brush onions on both sides with oil and season with salt and pepper. Grill on each side for 3 to 4 minutes or until slightly charred and just cooked through.04Whisk together vinegar, mustard, salt, pepper, and thyme in a medium bowl. Whisk in 1/4 cup of olive oil until emulsified. Add the onions and stir until combined. Let stand at room temperature until ready to use.