Thoroughly rinse canned kidney beans under cold water and pat dry to remove excess sodium and starch, which prevents the salad from becoming cloudy and allows the dressing to coat evenly.

Whisk together the mustard, lemon juice, apple cider vinegar, and garlic 15 minutes before adding vegetables so the flavors can meld and the garlic infuses into the acidic base.

Toss chopped red onions with a pinch of salt and lemon juice 10 minutes before assembly to soften their bite while adding brightness to the salad.

Fold in mint and fresh herbs just before serving to preserve their delicate flavors and prevent them from wilting or bruising in the acidic dressing.

Prepare the salad 2-4 hours ahead and refrigerate so the kidney beans absorb the dressing's flavors while the vegetables release their natural juices, creating a more cohesive dish.