Thinly slice the radishes into half-moons. Finely mince the onion, snip the chives into 1/4-inch pieces using kitchen shears (this is faster and cleaner than chopping), and roughly chop the mint. Measure out the feta cheese.
Bring a pot of water to a boil and add 1 tsp salt. Add the frozen peas and cook for exactly 5 minutes. Drain in a colander and set aside.
In a small saucepan, heat the olive oil over medium heat. Add the minced onion and sauté for about 5 minutes, stirring occasionally, until it softens and becomes translucent.
Remove the saucepan from heat and stir in the white wine vinegar. Pour the warm dressing over the cooked peas from Step 2, then gently fold in the sliced radishes, mint, and chives.
Transfer the salad to a serving dish or individual plates. Crumble the feta cheese over the top and season with the remaining 0.5 tsp salt and the pepper.
