Rigatoni with Spicy Calabrese-Style Pork Ragù

INGREDIENTS

• 1 pound mezzi rigatoni or penne rigate
• Kosher salt, freshly ground pepper
• 1 celery stalk, cut into 1" pieces
• 1 tablespoon tomato paste
• 1 pound hot or sweet Italian sausage, casings removed
• 1 carrot, peeled, cut into 1" pieces
• 2 teaspoons fresh oregano leaves
• 1/4 teaspoons crushed red pepper flakes
• 3/4 cup finely grated Parmesan or Grana Padano plus more
• 1 pound ground pork
• 1 28-ounce can whole peeled tomatoes
• 1/2 cup coarsely chopped flat-leaf parsley, divided
• 1/4 cup olive oil
• 1 medium onion, quartered
• 4 garlic cloves