Chop the cauliflower florets into bite-sized chunks and slice the radishes in half, discarding the tops and tips. Add the cauliflower and radishes to a small pan of boiling water. Simmer for 6 minutes. Add the peas and simmer for a minute or two more, or until all the vegetables are cooked and tender. Drain completely and set aside to steam dry, then transfer to a mixing bowl to cool.
Lightly crack the hard-boiled eggshells so you can easily peel the shell away. Roughly chop the eggs. Add the chopped eggs to the bowl with the cooked vegetables. Season with salt and pepper and mix well to combine.
Roughly chop the capers, parsley and gherkin. Add the capers, parsley and gherkin to the bowl with the cooked vegetables and egg. Mix well to combine.
Add the mayonnaise to bowl. Stir well to combine, coating everything evenly in the mayonnaise. Cover and chill until ready to serve.