6 tablespoons extra-virgin olive oil (EVOO), divided
1 large onion, finely chopped
Salt and pepper
3 tablespoons balsamic vinegar, divided (eyeball it)
1/2 tablespoon Dijon mustard
3 plum tomatoes, sliced
4 large cups baby spinach, arugula or a different green of your choice
1/4 cup blue cheese crumble
Preheat the oven to 325F.
Place hazelnuts on a baking sheet and put them in the oven to toast them up, about 10-12 minutes -- your nose will know when they are done. Remove them from the oven and cool. Run your knife through them once, chopping them coarsely.
While the nuts are toasting up, place a large skillet over medium-high heat with 2 turns of the pan of EVOO, about 2 tablespoons. Add the onions, season with a little salt and pepper, and cook, stirring frequently for 10 minutes. Add 1 tablespoon of the balsamic vinegar and a big splash of water. Use a wooden spoon to scrape up any bits on the bottom of the pan and cook for about 1 minute.
Remove the onions from the skillet and transfer to a bowl. Add remaining vinegar and mustard to the bowl and whisk in remaining EVOO in a slow steady steam.
In a salad bowl, combine tomatoes, greens, cooled hazelnuts, and the blue cheese crumble. Pour the caramelized onion dressing over the salad and toss to coat. Adjust the salt and pepper to taste.