In a large, cold skillet, add the oil, garlic and chile flakes. Cook over medium-low heat until golden. Add the onion, celery, carrot and salt and pepper. Add the sausage and cook until brown, breaking it into small pieces. Strain out any excess fat in the pan.
Add the crushed and diced tomatoes and bring to a boil. Season to taste with salt and pepper and simmer for about 10 minutes, stirring regularly.
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.
Meanwhile, make the pesto. Combine all of the 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. Divide between plates and top with pesto and parmesan.