Heat a Dutch oven or large saucepan over medium-high heat. Add oil and swirl to coat. Add onion and garlic to pan. Sauté 3 minutes or until onion starts is fragrant to brown. Add tomatoes, stock, oregano, and pasta, in that order. Bring to a boil. Stir to submerge noodles in liquid. Cover, reduce heat to medium-low, and cook 7 minutes or until pasta is almost done.

Uncover; stir in salt. Add spinach in batches, stirring until spinach wilts. Remove from heat and let stand 5 minutes. Sprinkle with cheese.