Combine the dry ingredients in a large mixing bowl. Add the wet ingredients, then whisk thoroughly to completely combine. You should be left with a smooth, slightly wet dough that you can just pick up.
Cover the bowl with a plate and leave for one hour to proof. Alternatively, cover and place into the fridge for 6 hours or up to overnight. While the dough is proofing, grease your baking dish thoroughly with a scant amount of butter and oil. Too much fat on the base can leave the focaccia with a soggy bottom.
When the dough is proofed, it should be light, fluffy and cohesive. Use wet hands to press it into the greased baking dish. Grease your fingers with oil, then use them to poke holes in the dough, right down to the base of the dish. When you’re happy, cover the dough and leave for another 30 minutes to proof again.
Preheat the oven to 200C/400F. When the focaccia is proofed, puffy and ready, cover it in the remaining olive oil and press in your toppings of choice. Finishing with sea salt flakes makes for an incredibly delicious top – I highly recommend it.
Put your focaccia in the oven for 25-30 minutes or until the top is crispy and firm. Turn the heat to grill and continue cooking until the top of the focaccia is golden brown. This might take 10-15 minutes. When you’re happy, remove it from the oven and allow to cool for 30-40 minutes before slicing. Reheat leftovers in a steamy oven or in the microwave to restore them to their former glory.