In a blender, combine the onion, garlic, the halved jalapeño, tomatoes with juices and ½ teaspoon each salt and pepper. Blend until smooth, about 1 minute, scraping the jar as needed; set aside.
In a large saucepan over medium-high, heat the oil until shimmering. Add the noodles and cook, tossing often with tongs, until golden brown, 3 to 4 minutes. Remove the pan from the heat. Carefully stir in the puree and the broth (the liquids may splatter). Bring to a simmer over medium-high and cook, uncovered and stirring occasionally, until the noodles are tender, 2 to 3 minutes.
Off heat, taste and season with salt and pepper. Serve garnished with the sliced jalapeño.
