Line a grill pan with foil and heat grill to medium. Beat the egg white on a plate and add a little salt & pepper. Tip the parmesan onto another plate & dip each chicken breast, first in egg and then the parmesan, until well coated. Grill the chicken for 10-15 minutes, turning once or twice, until cooked through, brown and crispy.
Meanwhile, boil the potatoes for approx 10 minutes until cooked, adding the peas for last 3 minutes of cooking time.
Drain the potatoes and peas and toss with the tomatoes, onion and spinach, adding the oil and vinegar, and season to taste.
Serve the salad on four warm plates, placing the cooked chicken on top, garnish with chopped mint, lemon zest and a little lemon juice.