Step 1

Preheat oven to 375 degrees F. Heat 1 1/2 tablespoons oil in a large cast-iron skillet over medium-high heat. Add mushrooms and onion; cook, stirring often, until the moisture released from the vegetables evaporates and the mushrooms are lightly browned, 9 to 10 minutes. Stir in flour; cook, stirring constantly, for 1 minute. Gradually add milk; cook, stirring constantly, until the liquid thickens, about 2 minutes. Stir in tarragon, salt and 1/4 cup Parmesan until melted. Stir in asparagus, rice and chicken. Remove from heat.

Step 2

Toss panko with the remaining 1 1/2 teaspoons oil and 1/4 cup Parmesan in a small bowl; sprinkle over the chicken mixture. Bake until the mixture is bubbly and the topping is golden, about 15 minutes.