1 (5-oz) bag baby spinach
4 oz large button mushrooms, wiped clean, sliced
1 carrot, peeled, cut into julienne strips
Red Onion Vinaigrette:
1 small red onion, halved top to bottom, cut into thin strips
2 Tbsp olive oil
2 cloves garlic, minced
1/4 cup balsamic vinegar
1 tsp sugar
1/4 tsp each coarse (kosher) salt and black pepper
In salad bowl, toss all salad ingredients; set aside.
In medium skillet, sauté red onion in olive oil over medium-high heat until onion is just slightly browned, about 5 minutes. Add garlic; sauté 1 minute, until fragrant. Add vinegar to skillet, scraping up any browned bits from bottom of skillet with wooden spoon.
Sprinkle sugar over onions; sauté 1 1/2 to 2 minutes, until liquid is reduced by half. Pour vinaigrette over salad. Season with salt and pepper; toss and serve.