Finely chop 1 medium yellow onion and mince 4 cloves garlic. Remove the casings from 1 pound sweet or hot Italian sausages if needed. Form the sausage into 1-inch-wide meatballs (about 1 heaping teaspoon each), wetting your hands with water as needed to keep the mixture from sticking, and place on a baking sheet. You should have about 38 meatballs.

Heat 1 tablespoon of the olive oil in a Dutch oven or large pot over medium heat until shimmering. Add half the meatballs in a single layer and pan-fry until browned on the bottom, about 3 minutes. Use tongs to carefully flip the meatballs and brown a second side, about 3 minutes more. Transfer to a large plate (the meatballs will not be fully cooked). Repeat with the remaining 1 tablespoon olive oil and remaining meatballs.

Add the onion to the now-empty pot and sauté until softened and translucent, 3 to 5 minutes. Add the garlic and sauté until fragrant, about 1 minute more. Pour in 4 cups low-sodium chicken broth and 1 can crushed tomatoes. Stir to combine, then bring to a boil.

Reduce the heat to maintain a simmer. Stir in the reserved meatballs and any accumulated juices on the plate, 1 cup dried ditalini or small shell pasta, ½ teaspoon kosher salt, and ¼ teaspoon black pepper. Simmer uncovered, stirring occasionally, until the meatballs and pasta are cooked through, about 10 minutes.

Stir in 5 ounces baby spinach (about 5 packed cups), a few handfuls at a time, until just wilted, 1 to 2 minutes. Serve garnished with grated Parmesan cheese.