Place the olive oil into a small saucepan. Smash and peel the garlic cloves, then add to the oil. Add in the pepper flakes and the basil, then place over medium-low heat. Once the mixture comes to a boil, simmer for 1-2 minutes, or until the basil has wilted. Remove from the heat and leave to infuse.

While the oil is infusing, make the spaghetti according to the packet instructions and cook until al dente. Reserve 1/2 a cup of pasta water to add to the sauce at the end.

Slice the onion in half through the root, and peel off the skin. Place the butter and some oil into a large, heavy-bottomed pot and place over medium-low heat. Allow the butter to melt, then place the onion half into the butter cut-side down. Cook for 8 minutes on medium-low heat, or until golden-brown on the cut side and the butter has browned.

Add the canned tomatoes to the butter and use a wooden spoon to gently break up the tomatoes. Once the tomato sauce has come to a boil, turn down the heat, season to taste with salt, cover with a tightly-fitting lid, and allow to simmer for 8 minutes over low heat.

Combine the basil oil with the tomato sauce and stir until combined. Add extra salt to taste, if needed. Add in the pasta water and allow the sauce to simmer for an additional 5 minutes, uncovered. Remove the onion halves and blend the sauce in the pot using an immersion blender.

Toss the pasta through the sauce until the sauce coats the pasta evenly. Serve with grated Parmesan cheese and a sprinkling of fresh basil leaves (optional).