1 small red onion
5 tablespoons red-wine vinegar, divided
2 medium cloves garlic, grated
1 teaspoon Dijon mustard
1 ½ teaspoons salt, divided
⅔ cup grapeseed oil, plus more for grill grates
½ teaspoon paprika
1 medium zucchini, sliced lengthwise into 1/2-inch-thick planks
1 small yellow bell pepper, cut into 2-inch-thick strips
2 large romaine lettuce hearts, halved lengthwise
3 (8 ounce) boneless, skinless chicken breasts
1 (5 ounce) package fresh baby spinach
2 small tomatoes, cut into 1-inch wedges.
First preheat the grill to medium-high (400°F to 450°F). Halve onion lengthwise through root end. Cut 1 half into 1-inch wedges; set aside. Thinly slice the other half. Place the onion slices in a small bowl then add 2 tablespoons of vinegar; toss to combine. Set aside to pickle, stirring occasionally, until ready to use. Drain before serving.
Whisk the following ingredients: garlic, mustard, 1 teaspoon salt and the remaining 3 tablespoons vinegar together in a large bowl until the salt dissolves. Slowly drizzle in oil and whisk until the dressing is creamy. Set aside 1/4 cup of dressing. Stir paprika into the remaining dressing in the bowl. Combine zucchini slices, bell pepper strips, romaine halves and the reserved onion wedges on a large rimmed baking sheet. Drizzle with 1/2 cup dressing; toss to coat well, making sure to get dressing between the lettuce leaves. Place chicken in the large bowl with the remaining dressing; toss to coat. Sprinkle with the remaining 1/2 teaspoon salt.
Oil the grill grates by holding an oil-soaked paper towel with tongs. Grill the chicken and the vegetables, uncovered, for 1 minute. Continue grilling until the romaine is slightly wilted, about 1 minute more; transfer the romaine to the baking sheet. Continue grilling the chicken and vegetables, covered and turning occasionally, until the vegetables are tender-crisp with grill marks and a thermometer inserted into the thickest portion of chicken registers 165°F, about 6 minutes for the vegetables and 8 to 10 minutes for the chicken. Return the grilled chicken and vegetables to the baking sheet with the romaine; let the chicken rest for 5 minutes before slicing. Cut the romaine and vegetables into 1-inch chunks.
Transfer the chopped romaine to a large bowl. Add spinach and the reserved 1/4 cup of dressing; toss well. Arrange the mixture on a large platter; top with sliced chicken, zucchini, bell pepper, tomatoes and pickled onions. Serve immediately.