1. Cook peas in a medium pot of boiling water until tender-crisp, 2 to 3 minutes. Drain, cool in ice water, and pat dry.
2. Whisk vinegar, chili paste, sugar, and oil in a large bowl to blend. Pour about 1/3 cup of the dressing into a small bowl. Toss peas, radishes, onion, and mizuna with dressing in large bowl. Transfer salad to plates and serve immediately, with salt and more dressing to add to taste.
*Made from red chiles, sugar, and fermented soybeans, this spicy-sweet chili paste is great in meat marinades, smeared in lettuce wraps, and stirred into soups. Buy it from an Asian market or from hmart.com; it keeps for months in the fridge. Find colorful radishes at farmers' markets and gourmet grocery stores. Get mizuna, a type of mustard green, at Asian markets and farmers' markets.