Place an oven rack in the top third of the oven and preheat oven to 500°F.
In a 12-inch, cast-iron skillet over medium heat, brown sausage, breaking it up as it cooks, until golden and caramelized. Add the onion, garlic and cubanelles; season with salt and pepper.
Spread the sausage and pepper mixture in an even layer across the bottom of the skillet. Top with cheeses and place in oven for 20 minutes, until golden brown and bubbly.
Remove from oven and pour excess oil off the top. Sprinkle with basil, sliced pepperoncini and toasted pine nuts. Serve with garlic bread crostini alongside for dipping.