Prepare the sponge: Combine fresh yeast and warm water in a bowl, stirring to dissolve the yeast.

Add bread flour to the yeast mixture and mix vigorously for 2-3 minutes until a smooth, elastic dough forms.

Scrape the sponge starter into a clear container, cover with plastic wrap, and let it rise until it begins to indent on top (6-8 hours at room temperature or overnight in the refrigerator after 1 hour of rising at room temp).

Prepare the dough: Combine yeast and warm water in a large bowl, stirring to dissolve the yeast.

Add the sponge to the yeast mixture and mix with your hands to break it up.

Add bread flour and whole wheat flour and mix, kneading in the bowl for 1 minute. Cover and rest for 20 minutes.

Add kosher salt and knead to incorporate.

Gradually add olive oil and milk, kneading until incorporated.

Knead the dough on a lightly floured surface until smooth, about 2-3 minutes.

Place the dough in a lightly oiled bowl, turn to coat, cover with plastic wrap, and let it rise until almost doubled in size (approximately 2 hours).

Line a baking sheet with parchment paper.

Loosen the risen dough, pour onto a floured surface, and divide into 9 equal pieces.

Shape each piece into a ball and place on the baking sheet, 4 inches apart.

Brush each piece with olive oil and gently press down to form a disk.

Cover with plastic wrap and let rise until almost doubled in size (approximately 1.5 hours).

Preheat oven to 425 degrees F.

Toss tomato slices with olive oil, salt, and pepper.

Arrange the tomatoes on the dough in a decorative fashion.

Press down on the dough in a few places to create dimples.

Lightly brush the dough with olive oil, allowing it to pool in the dimples.

Sprinkle with kosher salt.

Bake: Place the baking sheet in the oven, mist the loaves with water, and bake for 15 minutes.

Reduce heat to 350 degrees F and bake for 15-25 minutes longer, until golden brown and crusty.

Remove focaccia from the oven, brush with olive oil, and sprinkle with fresh herbs if desired.

Let cool on a cooling rack.