Skillet Spinach, Mushroom & Wild Rice Casserole

• 6 tablespoons unsalted butter, divided
• 2 (8-ounce) packages cremini mushrooms, sliced
• ¾ teaspoon salt, divided
• 2 leeks, thinly sliced and well rinsed
• 3 cloves garlic, finely chopped
• 2 teaspoons chopped fresh rosemary
• 2 teaspoons chopped fresh thyme, plus more for garnish
• ½ teaspoon ground pepper
• 3 tablespoons all-purpose flour
• 2½ cups whole milk
• ¼ teaspoon ground nutmeg
• 1 (10-ounce) package frozen chopped spinach, thawed and squeezed dry 
• 1 (5.2-ounce) package garlic-and-herb spreadable cheese (such as Boursin)
• 2 (8.5-ounce) packages microwaveable wild rice, cooked according to package directions
• 1 cup shredded Gruyère cheese

1. Preheat oven to 350°F with a rack in top third position (or 6 inches from heat source). 
2. Melt 3 tablespoons butter in a 10-inch cast-iron or other deep ovenproof skillet over medium-high heat. 
3. Add sliced mushrooms and ¼ teaspoon salt; cook, stirring occasionally, until browned, about 16 minutes. 
4. Transfer to a plate; do not wipe the skillet clean.
5. Melt the remaining 3 tablespoons butter in the skillet over medium heat. 
6. Add sliced leeks and ¼ teaspoon salt; cook, stirring often, until softened and slightly translucent, about 5 minutes. 
7. Add chopped garlic, 2 teaspoons each rosemary and thyme and ½ teaspoon pepper; cook, stirring constantly, until fragrant, about 1 minute. 
8. Add 3 tablespoons flour; cook, stirring constantly, for 1 minute, then gradually add 2½ cups milk (about ¾ cup at a time); cook, stirring often, until the sauce thickens and coats the back of a spoon, 4 to 5 minutes. 
9. Stir in ¼ teaspoon nutmeg and the remaining ¼ teaspoon salt.
10.  Add drained spinach and 1 (5.2-ounce) package garlic-and herb cheese; cook over medium-low heat, stirring constantly, until the cheese is melted and the spinach is evenly distributed, about 2 minutes. 
11. Remove from heat; fold in prepared rice and the cooked mushrooms until combined.
12.  Spread into an even layer; sprinkle with 1 cup Gruyère.
13. Bake until the cheese is melted, 10 to 12 minutes. Increase oven temperature to broil (do not remove skillet from oven) and broil until the cheese is browned, 2 to 3 minutes.  
14. Let rest for 10 minutes before serving; garnish with additional thyme, if desired.