Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the spaghetti and return to a boil. Cook the pasta uncovered, stirring occasionally until the pasta has cooked through but is still firm to the bite, about 12 minutes. Drain well in a colander set in the sink.
Meanwhile, heat the olive oil in a large saucepan over medium heat. Stir in the onion and green capsicum. Cook and stir until the onion has softened and turned translucent, about 5 minutes. Stir in the turkey and spaghetti sauce. Bring to a simmer over medium-high heat, then cover and reduce heat to medium-low. Cook until the sauce is hot.
Once the spaghetti has been cooked and drained, stir it into the hot sauce along with the mozzarella cheese. Stir until the cheese melts then serve.