1. Make the dip: Put yolk, mustard, 3/4 teaspoon salt, and 1 teaspoon lemon juice into a food processor; pulse to combine. With machine running, add oil, drop by drop at first, then in a slow, steady stream, until it has been incorporated and mixture is thick. (Alternatively, whisk together egg yolk, mustard, salt, and 1 teaspoon lemon juice in a medium bowl. Gradually whisk in oil.) If the yolk mixture separates, place 1 teaspoon warm water in a nonreactive bowl; gradually whisk yolk mixture into water.

2. Stir in remaining 1 tablespoon plus 2 teaspoons lemon juice, the orange zest and juice, anchovies, capers, and garlic; season with salt and pepper. Refrigerate at least 1 hour (up to overnight). (Dip makes about 1 cup.)

3. Prepare the artichokes: Fill a large bowl with water; squeeze 1/2 lemon into water, then add the rind. Working with 1 artichoke at a time, remove tough outer leaves, and trim stems. Cut in half lengthwise. Squeeze 1/2 lemon over artichokes, and rub with cut side of lemon. Transfer artichokes to lemon water.

4. Bring a medium pot of water to a boil. Add juice of remaining lemon and 1 teaspoon salt. Drain artichokes, and add to boiling water. Reduce heat, and simmer, partially covered, until tender, 6 to 8 minutes. Using a slotted spoon, transfer to a baking sheet lined with paper towels to drain. Let cool completely. Refrigerate, covered with plastic wrap, until ready to use (up to overnight).

5. Prepare the fiddleheads: Prepare an ice-water bath; set aside. Bring a medium pot of water to a boil. Add 1 tablespoon salt. Add fiddleheads, and cook until tender and bright green, about 3 minutes. Using a slotted spoon, transfer to ice-water bath to stop cooking. Drain. Refrigerate, covered with plastic wrap, until ready to use (up to overnight).

6. To serve, arrange vegetables on a platter or in bowls. Serve with dip.