For me, a good spaghetti carbonara is all about Parmesan, bacon and pasta--and "light" is not an adjective I'd use to describe any of those foods! So instead of substituting ingredients, I simply use less of them. Thanks to the combination of eggs and pasta cooking water, this dish still has the lusciousness and creaminess of a classic carbonara.
Bring a large pot of salted water to a boil. Add the spaghetti and cook according to the package directions. Drain, reserving 1 cup of the pasta cooking water. Return the pasta to the pot.
While the pasta is cooking, heat the oil in a large skillet over medium heat. Add the bacon and cook, stirring occasionally so it cooks evenly, for 5 minutes. Add the onions and garlic and cook, stirring occasionally, until the onions are soft, about 3 minutes.
Whisk together the eggs and cheese in a small bowl. Add it to the pasta and immediately (so it doesn't curdle) stir and toss together. Add the bacon, onions and 1/2 cup of the reserved pasta water (see Cook's Note). Toss to coat well and serve immediately.
The pasta will absorb some of the liquid as it sits, so if not serving right away, toss with the remaining 1/2 cup warm pasta water just before serving.