Stir together the first 3 ingredients and let sit for 15-20 minutes until bubbly.
Mix together the flours and cornstarch. Add the sourdough starter, yeast mixture, and salt. Mix on medium high speed 4-5 minutes, with the paddle attachment, until smooth.
Add up to 3/4 cup (1 tablespoon at a time) total of a combination of Bob's Red Mill 1 to 1 gluten free flour, brown rice flour and white rice flour. Mix in by hand until combined. Start with a tablespoon of flour at a time. The dough should still be soft and " shaggy". Mix in by hand until combined.
Cover and let rest 20-30 minutes.
Spread onto a oiled (with olive oil) stone pans or sheet pans or large cast iron skillet. Smooth with wet fingers.
Cover with a towel to rise for 1-1½ hours or until 1½ times.
Add indents every inch or inch and a half, with your finger tip. Then drizzle olive oil across the top. Cover and let rise 45 minutes to an hour.
Add your choice of toppings: Add 2-3 crushed garlic cloves in indents, then sliced tomatoes, olives, or herbs. Or add a decoratation or design with fruits and veggies. Decorate using fennel, (wishy) apricots pieces, rosemary sprigs, green onion stems, cut baby tomatoes (flowers), fennel (grass) and rocks (kalamata olives). Add parmesan cheese sprinkled across.
Drizzle more olive oil across both focaccia trays. Add coarse diamond salt if desired.
Bake at 425 at medium low rack and upper third rack for 10-15 minutes. Rotate the pans. Increase temperature to 450. Bake for 10-12 minutes more.
To brown the tops more, broil 4-5 inches from the burner. Cut into squares or into slices.