01 -
Combine olive oil, smashed garlic cloves, red pepper flakes, and basil leaves in a small saucepan over medium-low heat. Bring to a gentle boil, then reduce heat and simmer for 1 to 2 minutes until basil wilts. Remove from heat and set aside to infuse.
02 -
Prepare spaghetti according to package instructions until al dente. Reserve 1/2 cup of pasta cooking water before draining.
03 -
Slice onion in half through the root and peel. In a large heavy-bottom pot, melt butter with a bit of olive oil over medium-low heat. Place the onion half cut-side down and cook for 8 minutes until golden brown and butter is browned.
04 -
Add canned tomatoes to the pot with the browned onion and gently break up the tomatoes with a wooden spoon if needed. Bring to a boil, then reduce heat to low, season with salt, cover tightly and simmer for 8 minutes.
05 -
Stir the infused basil oil into the tomato sauce. Adjust salt if necessary, add reserved pasta water, and simmer uncovered for 5 minutes.
06 -
Remove onion halves and blend the sauce directly in the pot using an immersion blender until smooth. Toss cooked spaghetti with the sauce to coat evenly.
07 -
Plate the spaghetti and optionally garnish with grated Parmesan cheese and fresh basil leaves.