Peel and cut the potatoes into a small dice (about 1 cm). Bring a pot of water to a boil then salt and add the potatoes. Cook the potatoes about 8 minutes or until soft when pierced and almost cooked through.
Add the pasta and cook together with the potatoes following the package instructions (spaghetti n.3 take 5 minutes).
Stir 2-3 tablespoons of the pasta cooking water into the pesto - this makes it creamy and raises its temperature so you will not be adding it cold.
Drain the cooked pasta together with the potatoes and place back into the same pot. Pour in the pesto and stir to distribute being careful not to mash the potatoes. Add a generous pouring of olive oil to ease the process. Serve and top with grated parmesan.
Notes
If the final product is too dry feel free to add even more olive oil and parmesan. I generally have my pasta "drier" when I make it with pesto compared to other tomato or cream based sauces, but you should suit your taste. You can also only use 300 g pasta instead of 350, but then you will feed 3 instead of 4.
Different types of potatoes and variation in dice size can cause different cooking times than stated above. Before adding the pasta, the potatoes need to be half-done so they manage to cook through as the pasta cooks.
Please notice that the nutritional information provided is made by an online calculator and only meant to be used as a guideline.