In a medium heavy-bottomed skillet, melt butter over medium-high heat. Add onions. Cook until the onions are soft and have lost their moisture. It may be necessary to adjust the heat so that the onions do not brown. Reduce heat to medium-low and continue to cook so that the onions brown slowly, stirring frequently, until dark brown, but not burned, about 45 minutes in total cooking time. It may be necessary to adjust the heat to prevent them from burning.
Heat olive oil in a medium skillet. Crumble sausage into skillet. Cook over medium-high heat until lightly browned, stirring occasionally. Be sure not to let the meat dry out. Add cream and cook, stirring, for 2 minutes. Transfer to a large heatproof bowl. Break the sausage into marble-size pieces, if necessary. Set aside.
Bring a large pot of salted water to a boil. Add peas and onions to sausage in bowl. Add agnolotti to boiling water. Place bowl over pot of boiling water to warm through. Cook until al dente, according to package directions. Strain pasta and add to bowl with sausage mixture along with a few tablespoons of the boiling water. The sauce should not be wet or runny. The agnolotti should be well coated and shiny from the sauce. Transfer to serving dishes and season with pepper. Serve immediately.