01 -
Melt 2 tablespoons of butter with 2 tablespoons of olive oil in a large nonstick, high-sided skillet over medium-low heat. Add onion slices to the pan. Cook undisturbed for 15 minutes, then stir and continue cooking for another 15 minutes. Gauge the depth of caramelization and cook an additional 10–15 minutes or until deeply browned, stirring occasionally.
02 -
Increase heat to medium. Add garlic and cook for 1 minute. Stir in salt, pepper, tarragon, and thyme, cooking for 1 additional minute. Remove pan from heat and add brandy or white wine, then return to heat and cook until liquid evaporates, about 5 minutes. Stir in Worcestershire sauce and set onions aside.
03 -
In a separate medium pot or large saucepan over medium-high heat, add mushrooms without fat and cook until browned, about 5–10 minutes.
04 -
Add remaining butter and olive oil to the mushrooms. Once melted, stir in rice and cook for 5 minutes, allowing the grains to lightly brown. Add spinach and cook just until wilted, about 1 minute.
05 -
Add chicken stock, bring to a simmer, reduce heat to low, cover, and cook for 15 minutes.
06 -
Remove lid, stir in heavy cream and cooked chicken, heating gently just until warmed through.
07 -
Spray a 9x13x3-inch casserole dish with kitchen pan spray. Transfer the rice mixture to the dish and spread evenly. Layer the caramelized onions over the top.
08 -
Scatter shredded Gruyère cheese over onions. Broil in the oven until the cheese is melted, bubbling, and golden brown.
09 -
Serve immediately while hot.