Bring a large pot of generously salted water to a boil. Cook the gluten-free pasta according to package directions, but check it 1-2 minutes before the suggested time—gluten-free pasta can go from perfect to mushy very quickly.
Before draining, reserve 1 cup of the starchy pasta water. This is crucial for creating your silky sauce later! Drain the pasta but do not rinse it.
While the pasta cooks, heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Add the Italian sausage, breaking it up with a wooden spoon as it cooks until it's browned and cooked through, about 5-7 minutes.
Transfer the cooked sausage to a plate lined with paper towels to drain excess fat, but don't clean the skillet—those browned bits are flavor gold.
Reduce heat to medium and add the remaining 2 tablespoons of olive oil to the same skillet. Add the diced onion and cook until softened, about 3-4 minutes.
Add the minced garlic and red pepper flakes to the onions and cook for another 30 seconds until fragrant. Watch carefully so the garlic doesn't burn.
Pour in the chicken broth, using it to deglaze the pan by scraping up any browned bits from the bottom. Let it simmer and reduce by about half, approximately 3-4 minutes.
Return the cooked sausage to the skillet. Add the spinach in batches, stirring until wilted before adding more. This should take about 2 minutes total.
Add the cooked pasta directly to the skillet along with ¼ cup of the reserved pasta water. Toss everything together over medium heat until the pasta is well coated and the sauce begins to thicken slightly, about 1-2 minutes.
If the mixture seems dry, add more pasta water, a little at a time, until you reach your desired consistency.
Remove from heat and stir in the lemon juice, lemon zest, and fresh basil. Season with salt and pepper to taste.
Serve immediately, topped with dairy-free Parmesan if using, and a light drizzle of good olive oil.