Recipe found on Martha Stewart’s website
3 TB unsalted Butter
1/2 cup minced Onion
1/3 cup All-purpose Flour
4 cups Whole Milk, room temperature
Coarse Salt and freshly ground black Pepper
About 1 3/4 lb Spinach, trimmed, washed and cut crosswise into 1/2” thick strips
5 oz Gruyere Cheese, grated 2 cups
3/4 lb Elbow Macaroni, cooked according to package instructions
In a large saucepan, melt butter over medium. Add onion and cook until translucent, 6 minutes. Whisk in flour and cook, stirring frequently, until mixture is pale golden and has a slightly nutty aroma, about 2 minutes. Whisking constantly, add 2 cups milk. Add remaining 2 cups milk, raise heat to medium-high, and whisk until smooth; season with salt and pepper. Cook, whisking constantly until sauce comes to a boil, 8 to 10 minutes. reduce heat to low and simmer gently, stirring occasionally, until sauce thickens, 13 to 15 minutes. Add spin each and cook, stirring constantly, until wilted, about 3 minutes. Add cheese and stir until melted, about 2 minutes. Add cooked macaroni and stir to combine. Serve immediately.