Fill your glass with ice, add the sweet vermouth and Campari, then top up with the Prosecco.
Stir, then garnish with an orange slice or twist and serve.
Notes
If you want to serve your Negroni sbagliato neat, stir the vermouth and Campari with ice in a cocktail shaker, strain into a champagne flute or martini glass, then top up with the Prosecco and garnish before serving.