Finely chop the onion and garlic.
Thinly slice the fennel and, using a spiralizer, cut the courgette into long, thin strips.
Heat the oil in a frying pan and gently sauté the onion for 5 minutes. Add the garlic and fennel, then cook for an other 2-3 minutes, over low-heat and under cover, then add the lemon juice and zest.
Put a lid on and gently sauté (under cover) for about 10 minutes. Add some water if it’s getting too dry. When the fennel is soft, stir in the courgette and Nutritional yeast. Season with salt and pepper, then add the chilli flakes, if used. Add a drop of water too.
Put the lid on, and sauté for 3-5 minutes, until the courgette has wilted, but it’s not mushy.
Stir through the chopped dill and serve warm.