Set a large pot of salted water to boil. Chop up bacon, if relevant, into little chunks. Toss into a skillet or cast iron pan and cook til crispy, adding onions about halfway through and regularly draining fat. Sauté stirring occasionally, until soft and just turning to color, 7 to 10 minutes. Meanwhile, chop or chiffonade the greens.
Add a pinch of red pepper flakes to the onions and stir for 30 seconds. Transfer the bacon and onions out of the pan, leaving a bit of the fat. Add all of the greens. Cook over medium to medium-high heat until greens shrink way down—use tongs to flip the greens around and help them wilt. Season with a pinch of salt. Turn off the heat.
Drain pasta and immediately add to the pan with the greens. Add the bacon and onions (if relevant) back. Pour egg mixture into pan and toss with tongs to combine, turning heat to low or medium if greens have cooled way down. Season with salt and pepper to taste. Add more lemon or more of the reserved pasta cooking liquid to taste. Sprinkle pasta with more cheese if desired and serve immediately.