Spinach Mac and Cheese

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 spinach 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.