Heat the olive oil in a 12-inch skillet over medium heat. Add the diced onion and cook for 5 to 6 minutes, stirring occasionally, until soft and pale gold.

Add the sliced garlic, rosemary sprig, and red pepper flakes if using. Cook for 2 minutes over medium-low, stirring often, until the garlic is fragrant and just beginning to color.

Pour in the crushed tomatoes. Stir to combine and raise the heat to medium. Cook uncovered for 5 minutes, stirring occasionally, until the tomatoes reduce slightly and darken in color.

Add the drained cannellini beans, vegetable stock, and parmesan rind if using. Stir to combine and bring to a gentle simmer.

Using a wooden spoon, press 4 to 5 beans firmly against the side of the skillet until they mash into the sauce. This thickens the liquid without adding any starch.

Simmer uncovered for 10 to 12 minutes, stirring occasionally, until the sauce is thick enough to coat the back of a spoon and the beans are fully warmed through.

Remove the rosemary sprig and parmesan rind. Add the spinach in two batches, folding it in and waiting for each batch to wilt before adding the next, about 2 minutes total.

Taste and season with salt and black pepper. Remove from the heat and finish with a drizzle of raw extra-virgin olive oil.

Serve immediately, topped with freshly grated Parmigiano-Reggiano if desired.