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Close-up of a panini sandwich with arugula, melted cheese, and crispy toasted bread.

Caramelized Onion Focaccia Chicken Sandwich

This is not just a chicken sandwich. The bread is already layered with that deep onion flavor, then you add sautéed mushrooms, Gruyère, mustard aioli, and melty mozzarella, and suddenly it tastes like a French onion soup and a grilled cheese had a very good idea.
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Course: Main Course
Cuisine: French
Keyword: Focaccia
Prep Time: 10 minutes
Cook Time: 10 minutes
Total Time: 20 minutes
Servings: 6
Calories: 278kcal
Author: Lola Jay

Ingredients

  • 1 Trader Joe’s French Onion Focaccia sliced
  • 1½ cups cooked chicken sliced or shredded
  • 1 cup mushrooms sliced
  • ¼ teaspoon salt
  • ¼ teaspoon onion powder
  • ¼ teaspoon dried oregano
  • 1 garlic clove crushed
  • 1 tablespoon extra virgin olive oil
  • ⅓ cup Gruyère cheese shredded
  • ¼ cup shredded mozzarella
  • ⅓ cup arugula
  • 3 tablespoon mustard aioli or Dijon + mayo mixed together
US Customary - Metric

Instructions 

Toast the Bread

  • Preheat oven to 375°F.
  • Slice the focaccia horizontally and place cut-side up on a baking sheet.
    1 Trader Joe’s French Onion Focaccia
  • Toast for about 10 minutes, until lightly crisp on the edges but still soft inside.

Cook the Mushrooms

  • Heat olive oil in a skillet over medium heat.
  • Add sliced mushrooms and cook for 3–4 minutes until they release moisture.
    1 cup mushrooms, 1 tablespoon extra virgin olive oil
  • Stir in garlic, salt, onion powder and cook until mushrooms are tender and lightly golden.
    ¼ teaspoon salt, 1 garlic clove, ¼ teaspoon onion powder

Assemble and Melt

  • Spread mustard aioli evenly over the toasted focaccia.
    3 tablespoon mustard aioli
  • Layer cooked chicken over the bottom half.
    1½ cups cooked chicken
  • Sprinkle Gruyère, oregano, and mozzarella evenly on top.
    ⅓ cup Gruyère cheese, ¼ cup shredded mozzarella, ¼ teaspoon dried oregano
  • Return to the oven or place in a skillet and cook until the cheese is fully melted and bubbly.

Finish

  • Top with sautéed mushrooms and a handful of fresh arugula.
    ⅓ cup arugula
  • Close the sandwich, slice, and serve warm.

Notes

Nutrition facts are only estimates.

Storage & Reheating

  • Refrigerate: Store wrapped for up to 2 days.
  • Reheat: Warm in a skillet or oven to re-crisp. Avoid the microwave.

Nutrition

Serving: 1sandwich | Calories: 278kcal | Carbohydrates: 32g | Protein: 18g | Fat: 9g | Saturated Fat: 3g | Polyunsaturated Fat: 1g | Monounsaturated Fat: 2g | Cholesterol: 38mg | Sodium: 561mg | Potassium: 147mg | Fiber: 1g | Sugar: 1g | Vitamin A: 142IU | Vitamin C: 1mg | Calcium: 105mg | Iron: 1mg