For the starter sponge, mix the yeast in the water followed by the sugar and flour. Cover with a cling-film and leave to rise and froth for 45 minutes in a warm place.
Begin making the dough by mixing the yeast yeast and water and whisk into the sponge with the olive oil. Mix in the flour followed by the salt, cornmeal and whole wheat flour. Combine the whole wheat flour one cup at a time, using a spatula or a wooden spoon to fold it in. When the dough roughly comes together, scrape it out on a floured surface and knead for 8 to 10 minutes, until soft and velvety. Return to a lightly oiled bowl. Cover the bowl with cling-film and leave to rise in a warm place for 1 1/2 hours.
Grease and line a 12 x 17 inch sheet pan and spray cooking spray on the parchment too. Turn the dough out on the pan and pressing gently with your fingers, spread the sticky dough to fill the pan evenly.
Dampen the same towel, cover the pan and keep for 45 minutes in a warm place to rise further.
Meanwhile, gently warm the olive oil and stir in the rosemary. Remove from the heat in 30 seconds. Preheat the oven to 220 degrees C after the dough has been resting for 15 minutes.
Making light indentations with your fingers arrange the halved tomatoes and chorizo slices evenly all over the dough. Drizzle with the rosemary oil, sprinkle lightly with salt. Bake for 20-25 minutes, spraying the oven 3 times with water within the first 10 minutes, till the focaccia is crisp and golden. cool on a rack and serve. for a softer bread, cover it with a towel for 10 minutes after removing from the oven.