Meanwhile, warm 1 tablespoon of the butter in a medium saucepan over medium-low heat. When butter starts to bubble, add onion and cook for 3 - 4minutes, until translucent, stirring often.
Add the peas and water to the pan and cook 3 minutes, stirring. Stir in the ham and cook 2 minutes. Add the cream; bring to a low simmer and cook 5 minutes.
Drain the pasta; place in a large serving bowl.
Stir remaining 2 tablespoons butter into the cream mixture over low heat until melted.