3 red onions, sliced into thin half-moons

1/4 cup balsamic vinegar

1/2 cup olive oil, divided

1 1/2 teaspoons kosher salt, divided

1 teaspoon freshly ground black pepper, divided

2 large shallots, minced

3 tablespoons spicy brown mustard

1/4 cup red wine vinegar

2 heads red leaf, butterleaf, bibb, or looseleaf lettuce

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Toss sliced onions with balsamic vinegar, 1/4 cup olive oil, 1 teaspoon salt, and 1/2 teaspoon pepper.
Heat a large sauté pan over medium heat. Add the onions and cook slowly, stirring occasionally, for about 30 minutes until soft, glossy, and caramelized. If they start to crisp, reduce the heat. Set aside to cool slightly.
In a small bowl, whisk together minced shallots, spicy brown mustard, red wine vinegar, 1/2 teaspoon salt, and 1/2 teaspoon pepper.
Slowly drizzle in the remaining 1/4 cup olive oil while whisking to emulsify the dressing.
Place lettuce leaves in a large bowl. Drizzle with vinaigrette and toss gently to coat.
Top with caramelized onions and spoon over any remaining balsamic glaze from the pan. Serve immediately.

Add crumbled goat cheese or feta for a creamy element.

Sprinkle toasted walnuts or pecans for crunch.

Include sliced pears or dried cranberries for a sweet contrast.

Roast onions instead of caramelizing for a hands-off version.

Serve slightly warm for a cozy flavor balance.

Store caramelized onions separately for up to 3 days; assemble salad just before serving.