Begin by mixing the egg yolks and mustard in a bowl using a whisk. Add a pinch of salt and slowly pour in olive oil while whisking to a consistency similar to mayonnaise. Add garlic and mix to form an aioli and add the juice of a lemon. Add another pinch of salt and pepper for seasoning.
Slice the large focaccia in half. Moisten the inside with olive oil and place the oiled side onto the griddle to toast.
Spread the aioli onto the toasted focaccia. Spread the vegetables on one side of the focaccia. Slice the chilli and scatter on top of the veggies. Add pieces of basil to taste.
Place the second half of the focaccia on top. Slice to quarters and serve.