Measure all ingredients and prepare a sufficiently large pot in which the pasta and sauce can cook together. Peel the red onion, cut it in half and slice it into fine cubes or thin strips. Peel the garlic cloves and finely chop them with a sharp knife or press them through a garlic press. Wash the lemon under hot water, dry it and finely grate the yellow zest. Use only the outer zest, not the bitter white pith. Wash the cherry tomatoes, pat them dry and halve or quarter them depending on their size.

Heat the olive oil in the large pot over medium heat. Add the onion to the hot oil and sauté for two to three minutes, stirring occasionally, until translucent. Add the garlic and dried thyme and cook for about one minute. The garlic should become fragrant without browning.

Add the frozen leaf spinach directly to the pot. Place the uncooked pasta in the pot with the spinach and distribute it as evenly as possible. Pour in the cream and vegetable stock. Stir the ingredients carefully so that the pasta is mostly covered by the liquid. Season the mixture with a little black pepper. Use salt sparingly at first because the stock and Parmesan may already be salty.

Bring the contents of the pot to a boil over medium-high heat. Once the liquid is boiling, reduce the heat and gently simmer the pasta without fully covering the pot. Cook the pasta for about 12 to 15 minutes, stirring thoroughly every one to two minutes to prevent sticking. Check the liquid level during cooking. If the sauce thickens too quickly, add hot stock or water a little at a time.

Add the prepared cherry tomatoes to the pot about two minutes before the pasta has finished cooking. Gently fold in the tomatoes so they warm through while retaining their shape as much as possible. Check that the pasta is al dente and the spinach is completely heated through.

Remove the pot from the heat and stir the cream cheese into the hot pasta. Add the lemon zest and, if desired, the grated Parmesan. Stir everything thoroughly until a smooth and creamy sauce has formed. Season the one-pot spinach pasta with salt and black pepper to taste. If the sauce has become too thick, stir in a small splash of hot stock or water.

Divide the pasta directly from the pot among deep plates. Garnish with additional Parmesan, black pepper or fresh herbs if desired and serve immediately while hot.