Place the chicken strips in a non-reactive bowl and add a good sprinkling of spice mix and a splash of olive oil – the spicier you like your food, the more spice mix you should use. Season lightly with salt, then set aside for 30 minutes.
Meanwhile, make the guacamole. Bring a saucepan of lightly salted water to the boil and set a bowl of iced water in the sink. Add the frozen peas to the boiling water and blanch for 3–5 minutes until tender. Drain them well, then tip them into the iced water to stop the cooking and set the colour.
Drain the peas again and transfer to a blender or food processor with about 2 tablespoons of oil, the lime zest and juice and a splash of water. Season with salt and blend to make a chunky purée. Set aside.
Halve the avocados, remove the stones and peel them. Put the flesh in a nonreactive bowl and use a fork to coarsely mash. Add the pea purée, green chillies and spring onions, and season with salt. Cover the surface closely with clingfilm and chill for up to 2 hours until required.
To make the tomato salsa, put all the ingredients in a non-reactive bowl and season with salt and pepper. Set aside until required.
When you’re ready to cook the chicken, heat a large well-seasoned sauté or frying pan over a medium-high heat, then brush the surface with oil. When it is hot, add as many chicken strips as will fit without over-crowding the pan and fry, turning once, for 4 minutes, until they are cooked through and tender. Cook in batches, adding a little extra oil, if necessary. Remove the chicken from the pan and keep hot.
To serve, divide the shredded lettuce among the taco shells. Top each with a couple of spoonfuls of guacamole, followed by chicken. Add the tomato salsa and a dollop of crème fraîche to each, then sprinkle with sunflower seeds and serve.