Preheat the oven to 425 degrees F. Remove the stems from 24 large cremini mushrooms (finely chop the stems for the filling).
Wipe the caps clean with a damp paper towel, then toss with 2 tablespoons olive oil and season with salt . Arrange on a baking sheet.
Cook 4 ounces sweet Italian sausage (casing removed) in a skillet with olive oil over medium heat, breaking up the meat, until no longer pink, 3 minutes. Add 1/2 finely chopped fennel bulb and the chopped mushroom stems and cook until tender, 3 minutes. Add 2 chopped garlic cloves, season with red pepper flakes and cook 1 minute. Let cool, then mix with 1 cup breadcrumbs, 1/2 cup shredded fontina and 2 tablespoons each grated parmesan, chopped parsley and chopped green olives. Divide among the mushroom caps.
Bake until the mushrooms are tender and the filling is golden, 15 to 17 minutes. Let cool 5 minutes, then remove from the baking sheet.