Preheat the oven to 425 degrees. In a small bowl, mix together olive oil, mustard, lemon juice, and 1/4 teaspoon each salt and pepper.
Put salmon, skin side down, in a baking dish, and spoon mustard glaze over each piece of fish. Bake until salmon is just cooked through and no longer red, 6 to 8 minutes. Sprinkle with chopped dill, if desired, and serve.