If you're just serving four, save the leftovers for making Spaghettini and Sausage Frittata with Goat Cheese later in the week. Slideshow: More Fast Weekday Pastas
In a large pot of salted boiling water, cook the spaghettini until al dente. Drain, reserving 1/2 cup of the pasta cooking water.
Meanwhile, in a very large skillet, heat the olive oil until shimmering. Add the sausage, break it up into small pieces and cook over moderately high heat until browned, about 8 minutes. Add the garlic and cook over moderate heat until lightly browned, about 1 minute. Add the tomatoes and cook until their skins begin to split, about 2 minutes. Add the spinach and cook until wilted, about 2 minutes. Remove from the heat.
Using the back of a spoon, gently pop the tomatoes to release their juices. Add the spaghettini and the reserved pasta cooking water and toss over low heat until the pasta is coated, about 1 minute. Season with salt and pepper and serve.