Quarter radishes and coarsely chop greens and set aside. Place radishes and snap peas on a rimmed baking sheet.
Drizzle with two tablespoons (30 ml) oil, season with salt and toss to coat. Roast until tender but firm, about 15 to 18 minutes. Set aside to cool to room temperature.
Whisk together lemon juice, mustard, honey and Herbs de Provence in a small bowl. While whisking, slowly drizzle in remaining 1/4 cup (60 ml) olive oil. Season with salt and pepper to taste.
Toss together arugula, radish greens, mint leaves and walnuts with radishes and snap peas in a large bowl. Add dressing and toss to combine.