Bring a large pot of salted water to a boil. Add the 1½ lb baby potatoes and cook until fork tender, about 12-15 minutes, depending on the size of the potatoes. Drain well.
While the potatoes cook, bring a small pot of water to a boil. Blanch the 200 g snow peas for 1 minute, then transfer them immediately to a bowl of ice water. Drain and pat dry. Set aside.
In a small bowl, whisk together the 1 tablespoon wholegrain mustard, 1 tablespoon Dijon mustard, 1 tablespoon prepared horseradish, 2 tablespoons lemon juice, 4 tablespoons sunflower oil, 1 teaspoon honey, ½ teaspoon kosher salt and ¼ teaspoon black pepper until smooth.
Prepare the remaining vegetables: Thinly slice 6-8 radishes, thinly slice 3 green onions, and finely chop ¼ cup fresh dill. Set aside until you're ready to assemble the salad.
Add the warm potatoes to the dressing and toss gently to coat. Let them sit for 5-10 minutes.
Add the snow peas, radishes, green onions, and fresh dill. Toss gently until everything is coated.
Taste and adjust the seasoning. Add a little more lemon for brightness, or a pinch more salt if needed. Serve slightly warm or at room temperature.