Casseroles are a great make-ahead meal that can be portioned out for several days. If you’re following the neutropenic diet, just be sure to follow guidelines for eating and storing leftovers. If you don’t care for Gruyére, swap in your favorite pasteurized cheese.
- Thoroughly rinse fresh produce under warm running water for 20 seconds. Scrub to remove excess dirt.
- Preheat oven to 375 degrees. Grease a 2- or 3-quart casserole dish.
- Heat a very large skillet over medium-high heat. Add kale and water, cover, and cook, stirring occasionally, until kale wilts, 10 to 15 minutes. Once kale is wilted and water is absorbed, remove skillet from heat and use tongs to remove kale to a plate. Set aside.
- Using tongs, wipe skillet clean with paper towels. Return skillet to medium heat and add 2 tablespoons olive oil. Add mushrooms in a single layer. Cook for 2 minutes without stirring. When bottoms are caramelized, use tongs to turn mushrooms once and season with ¼ teaspoon salt and ½ teaspoon pepper. Continue cooking without stirring for about 5 minutes.
- Add butter to skillet and cook until it begins to brown. Reduce heat to low and add garlic, thyme, and nutmeg. Cook for about 10 seconds. Add cooked kale and toss to combine.
- Sprinkle flour over kale mixture and cook for 1 minute. Add whole milk and broth and, stirring, bring to a boil. Reduce heat and cook until thick, 2 to 3 minutes. Add cream and stir to combine.
- Remove from heat and stir in rice. Pour mixture into prepared dish.
- Using tongs, wipe skillet clean with paper towels. Add remaining 2 tablespoons olive oil and heat over medium-high. Add onions and salt and pepper to taste. Cook, stirring constantly, until onions begin to soften, about 5 minutes. Continue cooking until onions are golden brown, about 20 minutes.
- Sprinkle half of the cheese over the casserole, then spread onions in an even layer. Top with remaining cheese. Bake until cheese is melted and onions are crispy, 20 to 25 minutes. Casserole should register 145 degrees Fahrenheit or higher using an instant-read thermometer in the middle of the dish.