Cook pasta according to package directions. Drain in a colander over a bowl, reserving 2/3 cup cooking liquid.

While pasta cooks, heat oil in a large skillet over medium-high heat. Sauté onion 10 minutes or until lightly browned. Remove casings from sausage; discard. Add sausage to pan and cook 3 to 5 minutes or until browned, stirring frequently to crumble. 

Add garlic; cook 30 seconds, stirring constantly. Add tomatoes, salt, and pepper; cover and cook 3 minutes or until tomato skins split. Mash tomatoes with the back of a wooden spoon to break them up. Cover pan; reduce heat to low, and cook 3 minutes. Add pasta, reserved cooking liquid, salt, pepper, and spinach; toss well. Sprinkle with pecorino Romano. 

Note: Using cherry tomatoes helps to create a thicker sauce. If you substitute with grape tomatoes, cook for 5 minutes or until skins burst and use only 1/3 cup cooking liquid.