Put the smoked trout and softened butter in a food processor and pulse until smooth. Add the lemon juice, salt, and pepper and pulse again until combined. Taste and add more seasonings if necessary. Mix the Lemon Mayonnaise and the mustard together and transfer to a squeeze bottle. Pull the dill branches apart into small sprigs. Spread the pâté smoothly over the bread, lightly drizzle with the mayonnaise mixture, and garnish with the dill.
Refrigerate the remaining pâté and mayonnaise for future use. The pâté will keep for 1 week and the mayonnaise for 3 days. Bring to room temperature before using again.
Lemon Mayonnaise
1 tsp. (5 mL) Dijon mustard
1 egg
1 cup (240 mL) corn or canola oil
up to 3 Tbsp. (22.5 mL) lemon juice
¼ heaping tsp. (1.2 mL) kosher salt
1/8 tsp. (.5 mL) freshly ground white pepper
Put the mustard in a bowl and whisk in the egg yolk. Dribble the oil very slowly into the egg, whisking continuously. If the whisking makes your arm tired, take a break. The mixture will not separate.
When all the oil is incorporated, start adding the lemon juice half a teaspoon (2.5 mL) at a time. Continue to whisk. Taste after each addition. Add salt and pepper and taste again.
If this method seems too daunting, put the mustard and egg yolk in a food processor and pulse to blend. With the processor on, slowly pour in the oil. When the mixture has emulsified, add the lemon juice, salt, and pepper. Mayonnaise made in a food processor will be thicker than mayonnaise whisked by hand.
Cover and keep in the refrigerator until ready to use.