Add yeast and water to a large mixing bowl, and gently whisk. Let this rest uncovered for 5-minutes.
Add flours and salt to the same bowl. Using a wooden spoon, stir until a dough forms. Your dough will be wet and sticky, but combined.
Leave your dough in the bowl, and cover with a kitchen towel. Leave to rise for 90 minutes in a warm location, or until your dough has roughly doubled in size.
While your dough is rising, generously grease your 9x13 inch baking pan with olive oil. You don't have to be precise with this, but you'll likely use 2-3 Tbsp. Use your fingers to make sure the bottom and sides of the pan are well coated.
After 90 minutes, uncover your dough. Gently pull the edges of your dough toward the center until a smooth ball of dough forms. Transfer dough ball to the greased baking pan. Using your hands, gently stretch the dough until it fills the pan. Cover with your kitchen towel, and leave to rise in a warm location for 30 minutes.
While your dough is rising for the second time, position your oven rack to the middle and pre-heat to 425 F.
When your dough is done rising, remove the kitchen towel and drizzle with another 2-3 Tbsp of olive oil. Then, use your fingers to gently dimple the dough. Finish with a generous sprinkle of flakey sea salt.
Bake your focaccia for 25-30 minutes, or until the top is golden brown and slightly crisp. When it's done baking, allow it to completely cool in the pan before transferring to a cutting board to slice (note 3).