For Caramelized Onions:

• 3 tablespoons butter, divided
• 3 tablespoons extra virgin olive oil, divided
• 3 pounds sweet onion (about four large onions), cut in half and sliced with the grain into thick slices
• 1 tablespoon fresh garlic, minced
• 1 ½ teaspoons kosher salt
• ½ teaspoon freshly ground black pepper
• 1 teaspoon dry tarragon
• 1 teaspoon dry thyme
• 1/3 cup brandy or white wine
• 1 tablespoon Worcestershire sauce

For Rice Mixture:

• 8 ounces button mushrooms, sliced
• 1 ½ cups white uncooked rice
• 2 cups baby spinach, packed (about 2–3 ounces in weight)
• 3 cups chicken stock (from homemade or boxed)
• 1 cup heavy cream
• 3–4 cups cooked chicken torn into bite-sized pieces (from cooking your own chicken or rotisserie)
• 4 ounces Gruyere Swiss cheese, shredded