1. Remove the tough part of the asparagus; peel and rinse. Cut into small pieces. Slice the ends; set aside.
2. Peel the onion and chop it. In a Saucepan, melt butter, and cook the onion until it is translucent. Add the asparagus and continue cooking for a few minutes. Add 8 tablespoons of hot water and cook for 10 minutes over medium heat.
3. Put everything in a food processor and mix. Filter, then put the mixture back into the saucepan, adding salt; boil for a few minutes. In a large quantity of boiling salted water, cook the pasta al dente.
4. Pass the ricotta through a fine-mesh sieve. Drain the pasta and put it in a serving dish.
5. Add the ricotta, freshly ground pepper, and the asparagus tips.
6. Remove the saucepan from the heat. Combine the sauce with the pasta. Mix and serve immediately.