Bring a large pot of generously salted water to a boil. Cook the spaghetti until al dente, reserving 1 cup of pasta water before draining.
Meanwhile, heat a large skillet over medium heat. Add the butter. Once melted, add about 8 sage leaves and cook for 1–2 minutes until crisp. Remove the sage to a paper towel, leaving the butter in the pan.
Add the sausage and cook, breaking it into bite-sized pieces, until browned and cooked through, about 6–8 minutes. If there's excess grease, spoon off all but about 1 tablespoon.
Stir in the shallot and cook for 2–3 minutes until softened. Add the garlic, remaining chopped sage, and red pepper flakes, if using, and cook for another 30 seconds.
Add the corn and cook for 3–4 minutes until just tender and bright yellow.
Pour in the white wine and scrape up any browned bits from the bottom of the pan. Let it simmer for 2–3 minutes until reduced by about half.
Stir in the heavy cream and chicken broth. Bring to a gentle simmer for 3–4 minutes.
Reduce the heat to low and stir in the Parmesan until melted and smooth.
Add the cooked spaghetti along with the lemon zest and lemon juice. Toss until evenly coated, adding reserved pasta water a splash at a time until the sauce becomes glossy and clings to the pasta.
Taste and season with salt and plenty of black pepper as needed.
Top with the crispy sage leaves, and extra Parmesan before serving.