1 pound linguine
12 ounces bacon, cut crosswise into 1/2-inch slices
1 large red onion, halved, sliced (about 6 cups)
2 large bunches Swiss chard, stemmed, chopped (about 12 cups)
1 tablespoon balsamic vinegar
3 tablespoons extra-virgin olive oil
2/3 cup grated Parmesan cheese
Cook linguine in large pot of boiling salted water until tender but still firm to bite, stirring occasionally. Drain, reserving 1 cup cooking liquid.
Meanwhile, cook bacon in heavy large pot over medium heat until beginning to crisp, about 10 minutes. Let cool on paper towels. Drain all but 2 tablespoons bacon drippings from skillet. Add onions to drippings and sauté over medium-high heat until softened, about 7 minutes. Add Swiss chard and sprinkle with salt and pepper. Add pasta cooking liquid to skillet. Toss until chard is wilted and tender, about 4 minutes. Sprinkle vinegar over and cook 1 minute more.
Add linguine and oil to sauce in pot and toss to coat. Transfer to large bowl. Sprinkle with bacon and cheese. Season to taste with salt and pepper.