Pat chicken dry with paper towels and cut into steaks. To do this, place your hand flat on top of chicken breast and use a knife to slice through horizontally to make two thin steaks. Season with salt and pepper.
Heat a drizzle of oil in a small fry-pan on high heat. Cook chicken for about 30 seconds each side, until starting to brown, then place onto one side of the prepared tray (the chicken does not need to be cooked through yet).
Very finely chop walnuts. Brush chicken lightly with mustard and evenly coat with walnuts, pressing down firmly with fingers to make them stick.
Tear fold overs into 3cm pieces and place on tray next to the chicken. Drizzle with a little olive oil and bake for 5-7 minutes, until chicken is cooked through and bread is crispy. Remove tray from oven and set chicken aside to rest for 2-3 minutes.
While chicken and fold overs bake, dice tomato and cucumber 1cm and crumble feta. Place all in a large bowl, along with baby spinach and lemon juice. While chicken is resting, add croutons and a drizzle of olive oil to bowl with salad and toss to combine. Season to taste with salt and pepper.
TO SERVE divide walnut crusted chicken and baby spinach salad between plates and dollop with a little hummus.