Mix all the ingredients in a large mixing bowl using a wooden spoon. Scrape down the sides until you have a uniform, sticky dough.
Cover the dough with a kitchen towel or cling film.
Let it rise for 1 hour.
Line your baking tin with parchment paper.
Add 3-5 tablespoons of olive oil to cover the bottom of the tin.
Pour the dough into the tin.
Stretch and pull each side of the dough over the remaining dough a few times. You may need to put a tablespoon of olive oil on your fingers so it won't stick.
Stretch the dough carefully into each corner so it covers the entire form.
Cover with a clean kitchen towel or plastic wrap and rise for 1 hour.
Turn on your oven to 210°C/ 410°F.
Drizzle the focaccia with olive oil, and get some on your fingers.
Press your fingertips into the top of the dough to make the characteristic focaccia dimples.
Sprinkle generously with sea salt.
Melt the butter. Add minced garlic and chopped parsley.
Pour ¾ of the melted butter and garlic mixture on the unbaked focaccia.
Bake for 25 minutes or until the focaccia is golden brown.
Remove from the oven.
Spoon the remaining garlic butter over the baked focaccia.
Top with the grated Parmesan and cheddar.
Bake again until the cheese is bubbly and golden.
Remove from the oven.
Serve.