In a large skillet over medium-low heat, melt the butter and olive oil together. Add the sliced sweet onions, stirring occasionally, and cook for about 30 to 45 minutes until they become deeply caramelized and a rich golden-brown color.

Once the onions are caramelized, stir in the minced garlic, kosher salt, black pepper, dried tarragon, and dried thyme. Cook for an additional minute until fragrant, then deglaze the pan by pouring in the brandy or white wine.

In a separate pot, sauté the sliced button mushrooms over medium heat until tender. Then, add the remaining olive oil and butter. Stir in the uncooked white rice for about 5 minutes. Mix in the baby spinach and cook until just wilted, then pour in the chicken stock, cover, and simmer for about 15 minutes until the rice is tender.

Once the rice is cooked, stir in the heavy cream and cooked chicken pieces. Transfer the mixture to a greased casserole dish, spreading it evenly. Layer the caramelized onion mixture on top and sprinkle with shredded Gruyère cheese.

Preheat your oven's broiler. Broil the casserole for about 3-5 minutes until the cheese is melted and golden.