Heat oil in a large skillet or Dutch oven over medium heat. Add onion and cook, stirring constantly, until onion softens, about 5 minutes. Add garlic and cook another minute until fragrant.
Remove sausage meat from casing and add to Dutch oven. Break into small pieces with a wooden spoon. Cook, stirring occasionally, until sausage is brown, about 7 minutes.
Meanwhile, fill a large pot with water and heat over high. Toss in a handful of salt and bring to a boil. Cook pasta according to package directions. In the last minute of cooking time, add your peas.
Using a heatproof measuring cup, scoop up about 1 cup pasta cooking liquid. Add the cream to the measuring cup with the pasta liquid.
Add warmed cream to Dutch oven and stir constantly until cream cooks down a bit and a sauce forms. Remove from heat.
Drain fusilli and peas and pour into skillet and mix to coat with the sauce. Sprinkle in cheese and stir.
Serve with fresh basil.