Saute sausages in Dutch oven over high heat until brown, breaking into small pieces with back of spoon, about 12 minutes. Using slotted spoon, transfer sausage to bowl. Pour off all but 1 tablespoon drippings.
Add onion and saute until light brown, about 6 minutes.
Add cream; boil 5 minutes. Add broth; boil until reduced to sauce consistency, stirring occasionally, about 8 minutes. Return sausage to pot.
Cook penne in large pot of boiling salted water until pasta is just tender but still firm to bite.
Meanwhile, bring sauce to simmer over medium heat. Add peas and mascarpone and simmer until peas are tender, about 6 minutes.
Drain pasta. Add to sauce; toss to coat. Mix in Parmesan. Season with salt and pepper. Transfer to large bowl; serve.