In a large stock pot, heat the oil and garlic over medium heat. Toast until golden. Add the chile flakes and toast for 1 minute. Add the onion, celery, carrots, salt and pepper and cook until fragrant and soft.
Add the sausage and cook until brown, breaking it into smaller pieces. Drain out the excess fat.
Add the crushed and diced tomatoes and bring to a boil. Simmer for 10-15 minutes, until flavorful, stirring regularly. Adjust the seasoning to taste.
Meanwhite, bring a large pot of salted water to a boil. Add the pasta and cook according to package instructions. Drain the pasta, return to the pot, and drizzle with olive oil to prevent sticking.
Combine all of the pesto ingredients except the oil in a blender, food processor, or immersion blender. Process until mostly smooth. With the motor running, slowly add the oil until you reach a smooth and creamy consistency. Season to taste.
Toss the pasta with the ragu and divide between plates. Drizzle with pesto and parmesan to serve.