Melt 1 tablespoon of butter and 1 tablespoon of olive oil in a large nonstick skillet over medium-low heat.

Add the sliced onions and let them cook undisturbed for 15 minutes.

Stir occasionally and continue cooking for another 15 minutes until they become deeply caramelized.

Lower the heat to medium and introduce minced garlic, kosher salt, black pepper, dried tarragon, and thyme to the skillet. Sauté for about a minute.

Pour in the Worcestershire sauce and set the mixture aside for layering later.

In another saucepan, dry-cook the sliced mushrooms over medium heat for 5-10 minutes until golden brown.

Stir in the remaining 2 tablespoons of butter and olive oil into the mushroom pan.

Add the uncooked rice and toast for about 5 minutes until fragrant.

Gently fold in the baby spinach to the rice and mushrooms, allowing to cook down until it wilts.

Pour in the chicken stock, bringing everything to a boil.

Once boiling, cover the pan and lower the heat, letting it simmer for about 15 minutes until the rice is tender.

Once the rice is cooked, stir in the heavy cream and the torn pieces of chicken, heating everything through until warmed.

Grease a baking dish with butter or oil, pour in the mixture, layer the caramelized onions on top, followed by a sprinkle of shredded Gruyère cheese.

Place the baking dish under the broiler for a few minutes until the cheese is melted and golden brown.