Step 1

Mix anchovies, garlic, capers, vinegar, and mustard in a medium bowl. Add onion and olives and let sit, tossing occasionally, until onion is slightly softened, 8–10 minutes. Add 2 Tbsp. oil; season with salt and pepper. Toss to combine.

Step 2

Place tuna in a medium bowl. Zest and juice lemon over tuna. Stir in 2 Tbsp. oil; season with salt and pepper.

Step 3

Cut bread lengthwise without cutting all the way through. Add basil and parsley to olive mixture and toss to combine. Spread olive mixture over cut sides of baguette. Top with tuna, egg, tomato, and peppers, then drizzle with more oil; season with salt and pepper.

Step 4

Wrap sandwich tightly in foil and weight down with a heavy skillet, 10 minutes. Cut in half before serving.