In a large bowl, combine warm water, sugar, and yeast. Let it sit for 5-10 minutes until frothy. Add flour, salt, and olive oil. Mix until a dough forms. Knead for about 10 minutes until smooth. Place in a greased bowl, cover, and let rise for 1 hour or until doubled in size.

Bake the Focaccia

Preheat the oven to 425°F (220°C). Punch down the dough and spread it onto a greased baking sheet. Dimple the surface with your fingers and drizzle with olive oil. Bake for 20-25 minutes until golden brown. Allow to cool slightly before slicing.

Tuna Filling Preparation

Prepare the Tuna Filling

In a bowl, combine drained tuna, chopped olives, capers, mayonnaise, lemon juice, salt, and pepper. Mix well and adjust seasoning to taste.

Fennel Remoulade Preparation

Make the Fennel Remoulade

In a separate bowl, mix together finely chopped fennel, mayonnaise, Dijon mustard, lemon juice, salt, and pepper. Stir until well combined.

Assembly

Assemble the Sandwich

Slice the focaccia horizontally. Spread a layer of tuna filling on the bottom half, add sliced tomatoes and arugula, then drizzle with fennel remoulade. Place the top half of the focaccia on the sandwich and cut into individual portions.