Preheat the oven to 425 degrees Fahrenheit. Drizzle 2 tablespoons of oil over the salmon fillets and sprinkle some salt and pepper on them.
Place them skin-side up on a baking sheet with raised edges. Roast for 8 to 10 minutes or until the fish is flaky.
In a large nonstick skillet over medium-high heat, melt 1 tablespoon of butter with 2 tablespoons of oil. Season the halved radishes (not leaves) with salt and pepper and add them to the skillet.
Cook, stirring occasionally, until they are golden and slightly softened, about 8 minutes. Then, add 1 cup of water, the peas, capers, miso and mustard. Stir to dissolve the miso and mustard and cook until the peas are tender, 3 to 4 minutes.
Add remaining 2 tablespoons butter and simmer until a light sauce forms, about 4 minutes. Season with salt and pepper and garnish with dill.
Arrange the vegetables on plates or shallow bowls and place the salmon on top. Drizzle the sauce from the pan over the dish and enjoy it while it is warm.