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1
Preheat the oven to 400u00b0F.
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2
Combine the cherry tomatoes and red onion on a medium-size sheet pan. Drizzle the tomatoes and onion with balsamic vinegar and olive oil, and season them with with a pinch of salt and pepper. (You can add an extra splash of balsamic or olive oil if it's looking a little dry.) Toss everything on the sheet pan to combine (I like to use my hands).
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3
Wrap the garlic cloves in aluminum foil (they should make a little wrapped ball) and place on the sheet pan.
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4
Roast the tomatoes, onion, and garlic cloves in the oven for about 20 minutes, until the balsamic has reduced and caramelized the onions (they might be a little crispy on the edges) and the tomatoes are soft and blistered.
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5
Meanwhile, bring a pot of salted water to a boil. Cook the spaghetti al dente, according to package instructions. Drain in a colander, making sure to reserve a bit of pasta water.
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6
When the red onions and cherry tomatoes are ready, pull them out of the oven. Carefully unwrap the garlic gloves and, using a fork, mash them on the sheet pan until they are soft and broken up.
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7
Sprinkle the basil, plus another pinch of salt and pepper, over the sheet pan and toss to combine with the mashed garlic, tomatoes, and onion.
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8
Add the pasta to the sheet pan, plus a splash of pasta water and toss to combine. Serve warm, sprinkled with Parmigiano-Reggiano and extra basil sprigs, if you like.
