Put all dressing ingredients into a small screw-top jar with a tight--itting lid, adding a pinch of salt and pepper. Shake well and set aside.
Remove any wilted or bruised outer leaves on the lettuce. Separate the rest of the leaves, tearing out any thick stems; tear leaves into bite-size pieces. Wash the leaves, spin dry in a salad spinner, and transfer to a serving bowl.
Slice the carrots thickly on an angle. Halve the cucumber lengthwise and cut into half-moon slices. Slice the radishes as thinly as you can, and halve the tomatoes (or leave whole, if you like). Break up the broccoli florets into small bite-size pieces and add to bowl of lettuce along with the sliced beets. Gently toss together. Set aside.
Finely chop the herbs on a cutting board. Grate lemon zest over the herbs and use your knife to sweep everything into the middle of the board. Place the block of feta on top to coat it with the herbs. Drizzle with a little olive oil. Sprinkle with a pinch of salt and pepper; turn it over and coat the other side of the cheese with the herbs.
Toast the long slices of sourdough on a hot grill pan for a few minutes until golden with grill marks on both sides. When ready to serve, quickly dress the salad with half of the dressing and toss together. Serve the salad on the toasted bread with the herbed feta crumbled on top. Makes 4 servings.