Using a sharp knife, peel the carrot, and dice the celery finely.
Set a large frying pan over medium heat and when hot, tip in the oil then carrot and celery. Cook for 5 mins until starting to soften, stirring occasionally. Add the garlic oil, cook 2 mins more until the carrots and celery start to change color.
When the vegetables have 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 gluten free 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.