Bring a large, high-sided saucepan filled ⅔ with water to a boil. Add in the pasta along with 1-2 tsp of salt and cook the pasta until al dente, or according to instructions on the package.
In the meantime, heat up olive oil in a large frying pan on medium-high heat.
Add the sliced fennel, onion and the sugar to the frying pan and give it a good stir.
Gently fry while stirring regularly for about five minutes, until the fennel and onion are soft and evenly caramelised to a golden brown color. Turn down the heat a bit if the fennel and onion get dark brown too quickly.
Season with salt and pepper and stir in the lemon juice.
Fry for two more minutes, then deglaze the pan by adding the ¼ to ½ cup of starchy water from the pan of boiling pasta. (See note) If you've already drained the pasta at this point, just use tap water. Turn heat to low.
When the pasta is cooked, drain and add it into the frying pan. Turn off the heat and stir the pasta into the fennel and onion. Adjust sauciness with extra water if necessary.
Serve up and garnish with the reserved green fennel fronds.Optionally top the pasta with toasted pine nuts or sunflower seeds, or a sprinkle of vegan parmesan.
Pasta cooking waterWhy pasta water? Using pasta cooking water to deglaze the pan works great in this recipe because the starches dissolved in it act as a binder to create a smooth 'juice' to coat the pasta. Also the pasta water is already hot!If you've already drained the pasta and discarded the cooking water, no problem. Just use regular water.How much water? The amount of water you'll want to add can vary, and it's not lastly up to you how juicy or dry you would like the final dish to be. To start with, anything between ¼ and ½ cup (60-120 ml) will be fine for the 4 portion recipe.The hotter the pan and the longer you simmer down the liquid, the more water you have to add to end up with the same amount of 'pan sauce'.You can always adjust by adding more water or evaporating it off to reduce liquid. When the pasta is stirred in, it will soak up a bit more liquid, and you can still stir in a bit more water to add more moisture to the meal.