• 4 lettuce leaves or buns
To a pan set over the lowest heat, add 1 teaspoon oil, sliced onions 1/4 teaspoon salt and 1/4 teaspoon pepper. Cook for about 1 hour, stirring frequently until deeply golden and sticky.
During the last minute of cooking, add garlic and balsamic vinegar. Set aside to cool for 5 minutes.
In large bowl, combine ground chicken, puréed onion, spinach, artichoke hearts, mustard, cayenne pepper, 1 teaspoon salt, 1/2 teaspoon pepper and three-fourths of the caramelized onion mixture.
Preheat a cast iron skillet or pan over medium heat. Once pan is hot, add oil just to coat the bottom.