Modified from original: hot sausage only, breadcrumbs removed, fennel seed and red cracked pepper added. Original recipe by Annie Lucey.
Bring a large pot of generously salted water to a boil.
Trim tough broccolini stem ends (about 2 inches), discard, roughly chop remainder.
Heat olive oil in dutch oven over medium heat; add sausage, break apart, cook 5 minutes until lightly browned.
Move cooked sausage to pot edge; add onion with additional oil if needed and kosher salt; sauté until soft, stirring to lift browned bits.
Add garlic, fennel seed, and red cracked pepper; sauté about 1 minute until fragrant. Turn off heat.
Start with 2 tbsp each of fennel seed and red cracked pepper and adjust to your heat preference.
Add broccolini to boiling water, cook 2-3 minutes; remove with skimmer/strainer to colander. Save cooking water.
Return water to boil, cook pasta 2-3 minutes less than package instructions; reserve 2 cups pasta water before draining.
Add drained pasta to sausage pot with broccolini; cook over medium heat 2 minutes, stirring constantly.
Add 1½ cups reserved pasta water, lemon zest, lemon juice, and Parmesan; toss 1-2 minutes until creamy sauce forms. Season to taste; add remaining pasta water if needed for consistency.
Serve warm topped with additional Parmesan and cracked black pepper.