Heat the olive oil in a frypan over medium heat. Stir in the onion; cook and stir until the onion has softened and turned golden brown, about 10 minutes. Add the garlic and chilli flakes, stirring until fragrant, about 1 minute. Stir in the chicken stock, silverbeet and salt. Cover and reduce heat to low. Simmer until silverbeet is nearly tender, about 5 minutes. Remove lid and increase heat to medium, stirring until the liquid evaporates, about 5 minutes. Remove from heat and set aside.
Heat another frypan over medium-high heat. Warm the tortillas in the frypan for about 1 minute per side. Fill the warmed tortillas with the silverbeet and top with the cheese and salsa.