1 red onion, sliced thin
1 pint grape tomatoes
4 garlic cloves, finely chopped
⅔ cup olive oil, divided
Kosher salt and freshly ground black pepper
3 tablespoons red wine vinegar
1 teaspoon honey
1 teaspoon instant espresso powder (recommended brand Megdalia D’Oro)
Vegetable oil (for grill)
3 cups mixed greens, romaine, frisee, red leaf or lettuce of choice
1 cup arugula
½ English cucumber, halved lengthwise, seeds removed, thinly sliced
Combine onions, tomatoes, garlic, and ⅓ cup olive oil in a large resealable plastic bag; season with salt and pepper. Seal bag, pressing out air, and toss to coat. Let sit at least 1 hour.
Whisk vinegar, honey, and espresso in a medium bowl until honey and espresso are dissolved. Gradually add remaining ⅓ cup olive oil, whisking constantly. Season with salt and pepper; set aside.
Prepare grill for medium-high heat; lightly oil grates. (Or, heat a grill pan over medium-high.) Remove onions and tomatoes from marinade; grill, turning occasionally, until tender and charred in spots, about 5 minutes. Let cool. ALTERNATIVELY, preheat oven to 400 degrees. Place onions and tomatoes on a low-sided pan in a single layer. Roast for 15 – 20 minutes or until softened and slightly browned. .
Arrange greens on a platter. Season with salt and pepper and drizzle with half of vinaigrette. Top with cucumber, and grilled vegetables; drizzle with remaining vinaigrette.
Do Ahead: Vinaigrette can be made 2 days ahead. Cover and chill.