Finely dice the red bell pepper, cucumber, and tomatoes into small ¼ inch chunks. Finely chop the parsley and mint leaves. Grate the zucchini on the large edge of a box grater and roughly chop it further to form zucchini rice. Grate the lemon zest on the small edge of a box grater and juice half a lemon. To a large bowl, add the zucchini rice, tomatoes, cucumber, capsicums, parsley, peppermint, lemon zest, and scallions.
Gently toss to combine all the ingredients together. Drizzle the olive oil and lemon juice over the tabbouleh, season to taste, and give a final stir with a metal spoon. Sprinkle the pumpkin seeds and walnuts over the top before serving.