Heat oil in a pan over medium-high heat. Add the 500g minced meat and cook for 5 minutes, breaking it up as it cooks. Peel the 4 onions, cut into strips, add to the meat, and season with salt and the Mexican spice mix. Cook for 3 minutes.
Clean the 4 bell peppers and cut into large pieces. Add to the pan and cook for about 2 minutes so the peppers are tender but still have some bite.
Mix the 2 teaspoons cornstarch with the 250 ml water until smooth. Pour into the pan and bring to a boil while stirring.
Simmer for 1 minute until the sauce thickens slightly. Season to taste and serve.