Chop and prep all ingredients. Trim the ends off the green beans, no need to trim the pointy tips. Slice the bacon, mushrooms and onions.
Bring a pot of water to a rapid boil, add a pinch of salt and cook the pasta, about 12 minutes. In the last 3 minutes of the pasta cooking, add the green beans to the boiling pasta (Time the pasta for 9 minutes, then 3 more minutes together with the green beans). BEFORE DRAINING, use a coffee cup, ladle or measuring cup to pull out 1 to 2 cups of the starchy cooking water. Reserve it for the sauce. Drain the pasta and green beans together in a colander.
Meanwhile, while the pasta is cooking, heat a large skillet over medium heat and add the chopped raw bacon, the onions and the mushrooms. Stir often with a wooden spoon, the vegetables will cook in the rendering fat from the bacon, about 15 minutes.
When the bacon is fully cooked, the vegetables will be sautéed and golden brown. Add the garlic and stir a minute until fragrant. If there is excessive bacon fat in the skillet, you can spoon out some of it, if not, it will just become part of the sauce.
Add the heavy cream, reduce heat to medium low and bring to a simmer.
As the cream reduces, you can add the starchy pasta water to stretch the sauce. It will continue to reduce and thicken as it simmers. Give it a taste and add a pinch of salt and pepper if needed (the bacon may be enough salt for the dish).
Add the drained pasta and green beans to the sauce and toss well with tongs. Add more pasta water as needed if it becomes too dry. I added about ⅔ cup total.
Add fresh herbs and Parmesan cheese and continue tossing and serve!