Heat 1 tbsp oil in a wide, shallow saucepan or casserole dish over a medium heat. Season the chicken and tip it into the pan, fry for 5 mins, or until nicely browned but not necessarily cooked through. Transfer to a bowl and set aside.
Reduce the heat to low and heat the remaining oil in the pan, add the onion and cook for 8-10 mins, stirring now and then, until softened. Stir in the garlic and sizzle for a further 30 seconds.
Stir the mascarpone and pesto into the onions, crumble over the stock cube and add 500ml hot water. Stir to combine, bring to a gentle bubble, then add the chicken back to the pan.
Snap the lasagne sheets into smaller pieces, aiming to break each sheet into 3 or 4 shards. Add to the sauce a few pieces at a time to prevent the pieces from sticking together. Stir regularly for the first few minutes of cooking (approx. 3 mins), then add the spinach, pushing down into the sauce with the back of the spoon. Put the lid on and cook for 2-3 mins.
Remove the lid, give the lasagne another stir, the spinach should have wilted and the pasta sheets should be starting to soften. The sauce should still look quite loose at this stage, the liquid will be absorbed by the pasta as it cooks, but add a splash more water if it looks too dry or the pasta is sticking together. Stir every minute or so for the next 5 mins, or until the pasta is cooked through and the sauce clings to the pasta. Heat the grill.
Check the seasoning, adding more salt or some pepper if you like. Tear the mozzarella into chunks and arrange on top of the lasagne, scatter with parmesan, then put under the grill for 5-7 mins until golden and bubbling.
