Fry off your beef mince and onion until nicely browned and starting to crisp. Add your taco seasoning, fry a moment longer, season with salt and pepper and add your passata. Reduce slightly then remove from heat.
Butter two sides of your bread. Flip the slices and load with the two cheeses. Melt some butter in a frying pan and fry your sandwich until lightly coloured on each side and the cheese has melted. Place each sandwich on top of a rolling pin to form a taco shape. Leave to cool as such.
Place your 'tacos' in a serving dish, load with taco mince, then top with your toppings.