Halibut and Salmon Terrine with Aioli and Horseradish

INGREDIENTS

• For the Terrine
• 2 tablespoons extra-virgin olive oil
• 1 large sweet onion, chopped
• 12 ounces boneless, skinless halibut fillets, cut into chunks
• 12 ounces boneless, skinless wild salmon fillets, cut into chunks
• 2 large eggs
• 3/4 cup cold water, plus boiling water for pan
• 3 tablespoons matzo meal
• 1 tablespoon sugar
• 1/2 teaspoon fresh lemon juice
• Coarse salt and freshly ground pepper
• 1/2 large carrot, peeled and grated
• 1/2 large parsnip, peeled and grated
• 1 tablespoon chopped fresh dill
• For the Aioli (makes 2/3 cup)
• 1 garlic clove
• Coarse salt
• 1 large egg yolk, room temperature
• Juice of 1/2 lemon
• 1/2 cup light olive oil or peanut oil
• 1/2 cup extra-virgin olive oil
• Store-bought horseradish with beets, for serving