Peel and slice onions. Peel and roughly chop garlic.
Add oil to a large deep fry-pan (one you could comfortable add four serves of cooked pasta too). Heat to a medium.
Once the fry-pan is hot, add the sliced onions and sauté for 5 minutes.
Add garlic to the fry-pan at the 5 minute mark, turn the heat to a medium-low and sauté for another 5 minutes. In the last 2 minutes, mix in the tomato paste, dried oregano and chili flakes.
Add the vegetable stock. Bring to a boil, then reduce to a simmer. Remove from heat and let cool down (enough for your blender to safety handle the liquid).
Cook pasta, as per packet instructions.
Add the stock, drained cashews and sun-dried tomatoes (reserving 50g to toss through the pasta) to a food processor. Blend on the highest speed into a creamy sauce.
Drain pasta, reserving ½ cup pasta water.
Add spinach to the bottom of the frypan. Top with pasta, sliced basil and sliced remaining sun-dried tomatoes.
Toss until the spinach has softened, using a splash of pasta water to make it extra creamy.