Step 1

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.

Step 2

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 1/4 teaspoon red pepper flakes if desired and cook until fragrant, 30 seconds to 1 minute.

Step 3

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 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper and stir to combine. Bring to a simmer.

Step 4

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.

Step 5

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.