Thinly slice 1 red onion and 1 bunch radishes. Soak in ice water, 5 minutes; drain and pat dry. Whisk 4 teaspoons rice vinegar and 1/2 teaspoon each dijon mustard and sugar in a bowl. Whisk in 2 tablespoons vegetable oil and 1/2 teaspoon sesame oil. Add 8 ounces thinly sliced snow peas, the onion and radishes. Season with salt.