Preheat oven to 450 degrees Fahrenheit (230 degrees Celsius). Place artichoke hearts and portabella mushrooms in a 9×13-inch (23×33-cm) pan. Drizzle with olive oil and cook for 10–15 minutes, turning once midway through cooking. Allow to cool.

In a large bowl, combine baby spinach, red onion, and broiled vegetables. Combine dressing ingredients in a blender or by hand and pour over salad. Toss to combine.