Bring a large pot of generously salted water to a boil.
Trim the tougher ends of the broccolini stems off, about 2 inches or so, and discard. Roughly chop the rest of the broccolini, set aside.
While the water comes to a boil, in a heavy bottomed dutch oven or stock pot, heat olive oil over medium heat. Add the ground sausage, break up with a wooden spoon, and cook until lightly browned, about 5 minutes, stirring occasionally.
Use a wooden spoon to move the cooked sausage to the edge of the pot, and add onion. Add another glug of olive oil if needed and season generously with kosher salt. Saute the onions for several minutes until soft, stirring occasionally to lift up the caramelized brown bits in the pot. Add the garlic, and saute until fragrant, about a minute. The sausage should be golden brown, and the onions and garlic should be soft. Turn off the heat entirely and set aside.
When the water is boiling, add the broccolini and cook in water for about 2-3 minutes. Use a wire mesh skimmer, handheld strainer, or tongs, remove the broccolini from the water, and transfer into a colander to let the broccolini cool and set aside (save the water for your pasta).
Bring the water back up to a boil, and cook the pasta 2-3 minutes less than the package instructions, as you'll finish cooking the pasta in the other pot with the sausage. Before you drain the pasta, set aside 2 cups of the pasta cooking water, which makes the base of your sauce.
When the pasta is cooked, drain and add to pot with sausage, onion, garlic, as well as the cooked broccolini and cook over medium heat for about 2 minutes to incorporate all ingredients, stirring constantly.
Add 1 1/2 cups reserved pasta water, lemon zest, lemon juice, grated parmesan cheese, and toss for a minute or two, until ingredients are incorporated and the pasta water has reduced slightly to make a creamy sauce. Season with more lemon juice, grated parm, and kosher salt to taste. If pasta needs to be saucier, add the remaining 1/2 cup of reserved pasta water and stir until creamy.
Serve while warm and top with more grated parmesan cheese, and cracked black pepper.
Optional: Add crushed red chili flakes for a little heat and toasted breadcrumbs for crunch before serving.