Remove the leaves from the mint plant, wash and pat dry. Save several leaves for garnish. Tear the remaining leaves into small pieces. Set aside. Also wash the stems of the mint plant and set aside.
Place the ricotta in the serving bowl and mix in a few tablespoons of olive oil and some salt. With a wooden spoon, mix together vigorously for a few minutes until the ricotta becomes creamy. (This is easier to achieve with fresh ricotta.) Set aside.
Sauté the onion in a few tablespoons of olive oil and the butter. When the onion is translucent and soft, add the three-quarters of the chopped mint leaves and sauté together for about a minute, until the mint leaves are wilted. Remove from heat.
Meanwhile, bring a pot of salted water to boil for the pasta. Place the washed mint stems into the water and bring to a boil. When boiling, remove and discard the stems and add the pasta to the water. Cook the pasta until just before al dente. Drain the pasta, retaining a cup of the pasta cooking water.
Add the pasta to pan with the mint and onion mixture and cook together for about a minute over medium heat. Then, add the pasta and mint mixture to the ricotta in the serving bowl. Add some of the retained cooking water if the pasta doesn’t seem creamy. Mix in the remaining mint pieces, about 1/4 cup of Parmigiano and salt to taste. Serve immediately, garnished with the remaining mint leaves.