Heat the oven to 425°F.

Set a large oven-safe skillet or Dutch oven over medium heat on the stove.

Add 1 tablespoon of the olive oil to the skillet.

Add the Italian sausage to the skillet and break it into small pieces with a wooden spoon.

Cook the sausage for 5 to 6 minutes, stirring now and then, until it is browned in spots and no longer pink.

Use a slotted spoon to transfer the browned sausage to a plate, leaving the fat in the skillet.

Put the sliced fennel and sliced onion into the same skillet and add 1/2 teaspoon of the kosher salt.

Cook the fennel and onion over medium heat for 8 to 10 minutes, stirring often, until they are soft and lightly golden at the edges.

Add the sliced garlic, chopped rosemary, and crushed red pepper to the skillet.

Cook for 1 minute, stirring, until the garlic smells fragrant and no longer looks raw.

Pour the crushed tomatoes and chicken broth into the skillet.

Stir in the remaining 1/2 teaspoon kosher salt and the black pepper, then bring the sauce to a gentle simmer.