In a medium bowl combine the couscous with boiling stock or water, cover with a tight fitting lid or put a plate on it and let it stand for 10-15 minutes. Uncover and fuff with a fork, cool to room temperature.
Chop the mint, parsley and dill. Slice the cucumber, radishes and shallot, cut the cherry tomatoes in half.
In a large bowl combine the cooled couscous, herbs, sliced cucumber, radishes and shallots, add the halved cherry tomatoes and feta cheese, then toss gently to combine.
In a mason jar mix the lemon juice, Dijon mustard, grated garlic, salt, pepper and the olive oil. Put a lid on a jar and shake until the mixture emulsified. Pour over the salad and mix until it's evenly coated. Taste and add more salt if needed. Chill in a fridge before serving.