In stand mixer with dough hook attachment, combine flour and 2 ½ cups room temperature water. Mix on low making until incorporated. When dough becomes shaggy, remove attachment and cover bowl with cloth towel.

In small bowl, combine yeast, sugar and ½ cup warm water (between 100 to 110 degrees F.). Stir until yeast and sugar are dissolved. Set aside until yeast forms bubbles and becomes foamy, about 10 minutes.

Pour yeast into stand mixer with dough. Mix on low until dough absorbs all the liquid.

Add kosher salt and continue to mix on medium speed for about 5 minutes. The dough will be thick and sticky.

In large glass bowl, coat sides with 3 Tbsp. extra virgin olive oil. Into bowl, scrape dough and cover with cloth towel. Place in warm spot until dough has doubled in size, 2-3 hours.

Grease bottom and sides of 13x18-inch sheet pan with 2 Tbsp. olive oil.

Using large spatula or plastic bench scraper, fold dough a couple times in bowl to deflate it. Transfer to baking sheet.

With oiled hands, fold dough in half, rotate 90 degrees and fold in half again. Cover with oiled piece of plastic wrap and let rest for about 15 minutes.

With oiled hands, remove plastic wrap and gently stretch dough to fill baking sheet to edges. Cover completely with oiled plastic wrap and refrigerate for at least 8 hours, but no more than 24 hours.

When ready to bake, remove sheet pan from fridge and let sit in warm spot until dough becomes puffy and bubbly, about 1 hour. Preheat oven to 450 degrees F.

Using oiled hands, press your fingertips firmly into dough creating dimples all over.

Into dimples, alternate placing leeks and butternut squash; pull leaves off rosemary sprig and scatter evenly all over. Drizzle all over with olive oil and sprinkle with salt flakes.

Bake on center rack about 30 minutes or until deep golden-brown.

Let cool in pan. Use metal spatula to loosen focaccia from pan; cut into pieces in any size desired.