The beauty of a this sandwich is that it can be a catch all for anything you have along with a few classic ingredients, including fresh olive oil. It's made in advance to allow all the flavours to mingle; a sophisticated whole that stays intact when you bite into it. And guess what? No mayo!!!

2 anchovy fillets, minced (optional)
1 very small garlic clove, minced
1 tsp Sherry Reserva vinegar
½ tsp Dijon mustard
 Pinch of salt and freshly ground pepper
tbsp fresh extra virgin olive oil
1 tbsp Basil olive oil
1 round crusty country loaf or small ciabatta, halved
1 English cucumber, peeled, seeded and thinly sliced, crosswise 
1 medium ripe tomato or 4 cocktail tomatoes, sliced
½ small red onion, thinly sliced
1 jar tuna packed in olive oil, drained
2 tbsp chopped pitted olives, preferably a mix of black and green
1 hard-cooked egg, peeled and thinly sliced

In a small bowl, whisk together anchovies, garlic, vinegar, mustard, salt and pepper. Slowly drizzle in oils, whisking constantly. If using a country loaf, pull out some soft interior crumb to form a cavity. If using a ciabatta, you won't need to eliminate anything. Add sliced cucumber to vinaigrette and toss well. Spread half the cucumbers on bottom of bread. Top with tomato and onion slices, then with tuna, basil, olives and egg slices. Top egg with remaining cucumbers and vinaigrette. Cover with second bread half and firmly press sandwich together. Wrap tightly in foil or plastic wrap, then place in a plastic bag. Put sandwich under a weight such as a cast-iron frying pan topped with a filled kettle for 7 to 10 minutes, then flip and flatten for another 10 minutes. Or press by hand on both sides until flattened. Keep it wrapped for at least 8 hours before serving. The longer the pan bagnat rests, the better. 

Source: NYTIMES Cooking