1️⃣ Caramelize onions: In a large skillet, cook onions in butter & oil over medium-low heat for 30-45 minutes, stirring occasionally.
2️⃣ Add garlic, salt, pepper, tarragon, thyme, and cook for 1 minute. Pour in brandy (off heat) and cook until liquid evaporates. Stir in Worcestershire sauce and set aside.
3️⃣ Cook mushrooms in a dry pan until browned (5-10 mins). Add remaining butter & oil, then stir in rice and cook for 5 mins.
4️⃣ Stir in spinach and cook until wilted (1 min). Add chicken stock, bring to a simmer, cover, and cook for 15 mins.
5️⃣ Remove from heat, stir in cream and chicken, and transfer mixture to a greased 9×13-inch casserole dish.
6️⃣ Top with caramelized onions & Gruyère cheese. Broil until melted, golden, and bubbly.
7️⃣ Serve immediately and enjoy the deliciousness!