Bring a large pot of heavily salted water to a boil.
Meanwhile, dice 1 small yellow onion.
Mince 3 garlic cloves.
Remove the casings from 1 pound uncooked Italian sausage if needed.
Heat 2 tablespoons olive oil in a large frying pan or high-sided sauté pan over medium heat until shimmering.
Add the onion and sauté until softened and translucent, 3 to 4 minutes.
Add the garlic and ¼ teaspoon red pepper flakes if desired and cook until fragrant, 30 seconds to 1 minute.
Add the sausage and cook, breaking it up with a wooden spoon or spatula into small pieces, until cooked through, about 6 minutes.
Carefully pour in 1 (28-ounce) can crushed tomatoes.
Add ½ teaspoon kosher salt and ¼ teaspoon black pepper and stir to combine.
Bring to a simmer.
Reduce the heat to low and gently simmer, stirring occasionally, while you cook the pasta.
Add 1 pound dried short pasta to the boiling water and according to package directions until al dente.
Drain and return to the now-empty pot off the heat.
Add the sauce to the pasta and toss gently to combine.
Add 3 ounces baby spinach a handful or two at a time, stirring between each addition, until the spinach is wilted, 1 to 2 minutes.
Serve garnished generously with grated Parmesan cheese.
