Using a sharp knife, peel the onion, slice
in half through the root and trim off the top. Place the onion,
cut-side down, on a chopping board. With your knife parallel
to the board, cut 3 slices through the onion, keeping the root
intact. With knife point at root end, cut down through the onion,
from top to bottom 4 times. Holding the root end, finely slice
across the onion (see pic above), to give you small diced pieces.
Set a large frying pan over medium heat and
when hot, tip in the oil then onion. Cook for 5 mins until
starting to soften, stirring occasionally. Add the garlic, cook
2 mins more until the onions start to turn golden. When the
onion has softened, tip in the tomatoes and half the basil.
Leave to gently bubble for 15 mins until the sauce has
thickened and looks pulpy. Stir occasionally and break up any
large clumps of tomato with the back of your spoon.
Pour water ¾ of the way up your largest pan.
Heat over a high heat and add plenty of salt. When water has
reached a rolling boil, place the spaghetti into the pan. As it
softens, use your hands to push all of it into the water. Give it
a stir to separate the pasta, then cook according to pack
instructions, about 10 mins. Taste a strand of pasta. It should
be just soft, but not mushy. Scoop out a cup of water before
draining in a colander (this will help to loosen your sauce).
Finishing touches: Tip the drained pasta back into its cooking
pan, then pour over the tomato sauce. Give everything a good
stir. The sauce should just coat the pasta, if it is thick and looks
dry, stir in a few spoons of the pasta water. If it is watery, cook
over a low heat until evaporated, stirring often. Use your hands
to break the mozzarella into chunky pieces and stir through the
pasta along with the remaining basil leaves. Serve straight away.