Blend the pesto until it reaches a spreadable consistency
Melt the butter in a large non-stick saucepan over medium heat
Whisk in the flour to form a smooth paste then remove from the heat
Slowly add the milk while whisking continuously until smooth
Return to the heat and whisk until it has formed a thick sauce, season with salt and pepper
Preheat the oven to 180°C
Cover the bottom of a 30cm x 20cm ovenproof dish with a thin layer of white sauce
Layer pasta sheets over the sauce
Spread the pasta with about 1.5 tablespoons of pesto
Follow with a layer of white sauce, mozzarella, and Parmesan
Repeat the layers 3-4 times depending on dish depth
Brush sage leaves with oil and arrange on top
Bake for 20-25 minutes until golden and bubbling
Leave to stand for 5-10 minutes before serving
