thanksgiving's best green bean casserole

This is my father-in-law's favorite green bean casserole dish that he asks me to bring every Thanksgiving. It's a bit of work, so I always make it the day before Thanksgiving and it's so good, it's totally worth it. Like several of my recipes, this one started from Martha Stewart and has some adaptations I've made over the years. 

INGREDIENTS

• 6 tablespoons unsalted butter
• 1/2 an onion, cut into 1/4-inch dice
• 1 red bell pepper, seeded and diced
• 1 pound button mushrooms, stems trimmed, quartered
• 2 teaspoons coarse salt
• 1/2 teaspoon freshly ground pepper
• 1 1/2 pounds green beans, trimmed and cut into 2-inch pieces (snap 'em with your hands and save time)
• 1/4 cup all-purpose flour
• 2 cups milk
• 1 pinch cayenne pepper
• 1 pinch grated nutmeg
• 1 cup grated Parmesan cheese (don't buy pre-shredded; they add a starch that causes clumping)

INSTRUCTIONS​

1. In a large skillet over medium heat, melt 2 tablespoons butter. Add onion, and saute until it begins to soften, about 4 minutes. Add bell pepper and mushrooms, and cook until softened and most of the liquid has evaporated, about 8 minutes. Season with 1 teaspoon salt and 1/4 teaspoon pepper. Set aside to cool.
2. Prepare an ice bath: Fill a large bowl with ice and water; set aside. Bring a saucepan of water to a boil. Add beans, and cook until bright green and just tender, 4 to 5 minutes. Drain, and plunge into ice bath to stop cooking.
3. When cooled, toss drained beans with mushroom mixture; set aside.
4. Melt the remaining 4 tablespoons butter in a medium saucepan over medium-low heat. Add 1/4 cup flour, whisk constantly until mixture begins to turn golden, about 2 minutes. Pour in milk, and continue whisking until mixture has thickened, about 3 minutes. Stir in cayenne, nutmeg, and the remaining teaspoon salt and 1/4 teaspoon pepper. Remove from heat, and let cool to room temperature, stirring occasionally. Pour over beans, and toss to combine.
5. Butter a 9-by-13-inch glass or ceramic baking pan. Spread half the green-bean mixture over the bottom. Sprinkle on half the grated Parmesan, and spread with the remaining green beans. Combine the remaining Parmesan and the breadcrumbs, and sprinkle over casserole.
6. Cover with foil, and refrigerate until just before serving.
7. You can fry some onions to place on top if you wanted to... but no one wants them at our house, so I never do. 
8. Heat broiler, positioning rack about 8 inches from heat. Cook casserole, covered, until mixture is bubbly and heated through, about 10 minutes. Uncover, and cook until top is golden brown, about 30 seconds. Sprinkle fried shallots over top, and serve immediately.