In a large mixing bowl, whisk the yeast, sugar, and warm water. Set aside for 10 minutes, until it's foamy on top. (If it's not foamy, check your yeast packet, as it might be expired.)
Add the olive oil and stir it into the foamy yeast mixture.
Add the flour, salt, and oregano, and mix for a minute with a large wooden spoon or silicone spatula until it forms a soft, sticky dough.
Cover the mixing bowl tightly with plastic wrap (to retain moisture so it doesn't dry out) and let it rise at room temperature for 1 hour. The dough should begin to rise & grow in size, but it probably won't double by the time it goes into the fridge.
Transfer the bowl to the fridge for 12-16 hours (for slow, cold fermentation).
Shape and Proof the Dough
Spread three tablespoons of the olive oil on the bottom and up the sides of a 9x13 baking dish.
Dump the dough from your mixing bowl onto the center of the baking dish and press it down into all the edges and corners.
Cover the baking dish with plastic wrap and let it proof at room temperature for about 4 hours, or until the dough looks really puffed up and jiggles when you shake the baking dish.
Prepare the Tomatoes
Strain the canned diced tomatoes through a fine mesh sieve. Leave it over a bowl to continue straining out the juice for at least 20 minutes.
Add the Toppings and Bake
Preheat the oven to 450°F (230°C).
Spoon the strained tomatoes evenly over the dough. Sprinkle the minced garlic, oregano, and shredded mozzarella evenly on top. Crumble the Fontina into small pieces all over.
Generously coat your fingers in olive oil. Use your fingers to press dimples into the dough, pressing all the way to the bottom of the baking dish.
Don't skip oiling your fingers, otherwise the dough will stick to you when you try to make the indentations. And aim for the dimples to be around 1-inch apart from each other. like how you'd hold your hands if you were playing the piano.
Drizzle two tablespoons of olive oil evenly over the dough.
Bake for 18-22 minutes, or until the focaccia is puffed up and golden. Cool the bread for at least 20 minutes, then slice and serve.
Notes
Storage: Cool your bread completely & store leftovers in an airtight container at room temperature for up to 2 days, or in the fridge for up to 4 days.
Reheating: Skip the microwave, which can make the bread soggy. Instead I warm them in the oven on a baking sheet at 375°F for 5-10 minutes.
Toppings: Honestly this pizza focaccia is great as-is from the oven, but I like to drizzle chili oil crisp on top for a little heat, or add some chopped fresh herbs like basil & more oregano.