Bring a large pot of water to the boil. Add the pasta and cook for 90 seconds less than packet instructions.
Preheat the oven to 180 degrees C.
In a large saucepan, melt the butter over a medium heat. Whisk in the flour. Turn off the heat and gradually whisk in the milk. Turn the heat back on, and gently whisk until it has thickened.
Stir in the pesto. Tear the basil leaves with your hands and add those too. Stir in the peas. Turn off the heat.
Drain the pasta and tip into a large ovenproof dish. Pour the sauce over the top and mix until all of the pasta is coated in sauce. Dot the mozzarella over the top.
Bake in the oven for around 30 minutes until golden and bubbling.