Salt the water for the pasta well and cook the pasta according to the instructions on the packet.
Wash the baby spinach well and allow it to drain, then chop it up together with the walnuts, the clove of garlic, the parmesan, salt, pepper and the lemon juice in a blender (or hand mixer, it is just a little more involved). When everything is chopped well, add the olive oil and mix until it is smooth.
Drain the pasta water, except for about 1/2 cup (100 ml), and then return the pasta to the hot pot. Add the pesto and stir on a low heat together with the cream and the mozzarella cut into small cubes for about 1 - 2 minutes, until the pesto sticks well to the pasta and there is no liquid or cream left. If necessary, add salt.
Serve immediately and sprinkle with fresh pepper and some parmesan.