Step 3
While the spinach cools, make the white sauce. Melt butter in a saucepan, then sift in the flour (this helps reduce lumps). Whisk the flour and butter together over medium heat, and cook this paste for 2 minutes. Gradually pour in the milk, whisking well between each addition. Cook for a further 2 to 5 minutes, until the sauce thickens. Season with salt and pepper.