With the vegetables prepped, I can get to cooking, starting with the bacon. I until the meaty parts are crisp and the fat is tender. Keeping the slices whole, rather than cutting them into bite-size pieces before cooking, helps keep them from getting too crunchy and also makes for less babysitting while the bacon cooks—it's a lot easier to turn six pieces of bacon than it is to constantly stir a bunch of slivers around in a skillet.
The light-colored spring onion parts and fennel go straight into that same skillet, along with a splash of water and a sprinkling of salt, which helps draw out moisture and kick-start the softening process. I like to use a straight-sided sauté pan for this dish—it keeps the moisture released by the vegetables contained, and I don't run into the problem of onions riding up on the lip of the pan and browning, as can happen with a traditional skillet.
Once the onion whites and fennel are soft, I add in the green onion tops and cook them for a couple of minutes, until they're just starting to wilt. At that point, I stir in a cup of heavy cream and bring the whole mixture to a simmer, allowing it to thicken slightly. Now it's time to buzz it all up into a smooth purée. Because the vegetables are so soft, the blending process is quick, which keeps the mixture from overheating and dulling the color. Because we'll be finishing the pasta in the sauté pan, I pour the purée back into the pan, passing it through a fine-mesh strainer along the way for extra smoothness..