• 2 Tbsp. extra-virgin olive oil
• 2 Tbsp. unsalted butter
• 2 lb. onions (about 3 large), thinly sliced
• 2 garlic cloves, thinly sliced
• 1/4 tsp. crushed red pepper flakes (optional)
• 1 1/2 tsp. kosher salt, plus more
• 12 oz. linguine or other long pasta
• 4 oz. Parmesan, finely grated (about 1 cup), divided, plus more for serving
• ½ cup parsley, finely chopped (optional), plus more for serving
• Freshly ground black pepper