Heat up the grill and cook the sausages. You can also cook them in a pan on top of the stove. After they are cooked and cool enough to handle, cut them up into ¼ inch slices and reserve.
Heat up a large pot of water and bring it to a boil. Add a teaspoon of salt then add the pasta and let it cook until al dente. Be sure to reserve a cup of the pasta water before you drain the pasta in case you need it for the sauce.
Heat up a large fry pan big enough to hold the sauce and the past over medium high heat. Add the oil and when it’s hot, add the onion. Sauté for a few minutes until it begins to soften, then add the garlic. Let this cook for a minute or two then add the mushrooms.
Add the crushed red pepper and let the mushrooms cook until they loose all their liquids and start to brown. This should take about 10 minutes.
Add the chicken stock and fresh thyme and top with fresh spinach leaves. Toss and let this cook until the spinach wilts, just a couple of minutes. If you don’t think there will be enough sauce, add a little of the reserved pasta water. I like to add some because the water has starch in it from the pasta and helps the sauce stick to the pasta.
Add the sausage slices and stir. Hopefully the pasta is ready at this time and should be added to the sauce and stirred to coat.
Add the grated mozzarella cheese and stir into the pasta until it melts and both the cheese and sauce coat the pasta.
Taste and adjust seasonings with salt and pepper.