Cook the pasta according to the packet instructions, making sure not to overcook it. The pasta should be al dente (firm to the bite). Before draining, scoop out ½ cup of the pasta water and set it aside.
Heat 1 tablespoon of olive oil in a large, deep frying pan over medium–high heat. Add the sausages and cook just until lightly seared (they don’t need to be cooked through at this stage, only firm enough to hold their shape). Transfer the sausages to a chopping board, then slice them into rounds, about 1 cm / 0.4 inches thick.
In the same frying pan, add the remaining oil and heat over medium–high heat. Add the sliced sausages and cook until browned and cooked through. Push the sausages to the side of the pan, then add the butter. Once melted, add the mushrooms and cook, stirring occasionally, until golden.
Stir in the garlic, onion, fennel seeds, dried chillies and fresh chilli. Cook for about 2 minutes, stirring gently, until fragrant.
Pour in the wine and stir gently until the alcohol smell disappears. Add the stock and spinach, then cook until the spinach has wilted. Stir in the pasta and cook until heated through.
Add a few tablespoons of the reserved pasta water, adding more if the dish looks dry. Taste and season with salt and pepper, if needed. Remove the pan from the heat and stir in the parmesan. Serve immediately with extra parmesan, if desired.