Pre-heat your oven to 220ºC/200ºC fan.
Slice your peppers (2 red, 1 green) into thin strips. Peel and thinly slice the 2 onions. Bash the 4 garlic cloves with a rolling pin or pestle and mortar (just enough to break the skin of the cloves). Slice the lemon.
Place all of the vegetables (and lemon) into a large baking tray, add the olive oil (2 tsp) and 2 tsp of the cajun spice mix. Toss everything together so that the vegetables are coated in the oil and spices.
Place into the oven for 15 minutes.
Meanwhile cut the 4 lamb steaks into thin strips and toss the lamb strips in the remaining 1 tsp of cajun spice mix.
Once the vegetables have been in the oven for 15 minutes, add the lamb. Toss the vegetables and lamb together and return to the oven for 10 minutes.
Just before the fajita filling is ready, warm the flour tortillas according to the packet instructions and prepare any toppings.
Remove the fajita filling from the oven, remove the lemon pieces and garlic and serve.