Dissolve the yeast in lukewarm milk with 1 tablespoon of sugar and 1 cup of flour in a large bowl.

Let the yeast mixture stand in a warm place for about 15 minutes to activate.

Warm 1/4 cup of olive oil with rosemary and lemon zest in a small saucepan.

Add grapes and raisins to the infused oil; mix well.

Incorporate half of the grape-raisin mixture into the yeast mixture.

Mix another 1 cup of flour into the yeast mixture using a dough hook attachment.

Knead the dough until smooth.

With the mixer running, add salt and the remaining 3 cups of flour gradually, kneading until smooth after each addition.

Continue kneading for an additional 6 minutes.

Shape the dough into a ball on a floured surface and place it in an oiled bowl.

Cover with a damp towel and let it rise in a warm place until doubled in bulk, approximately 45 minutes.

Punch down the dough and lightly flour the work surface.

Turn out the dough and knead lightly.

Preheat the oven to 400 degrees F.

Oil an 11x17-inch baking sheet.

Press the dough into a flat disk with the heel of your hand.

Use your fingertips to nudge the dough into a rectangle.

Transfer the dough to the prepared baking sheet and brush with the remaining olive oil.

Let the dough rise again until doubled, around 30-40 minutes.

Make indentations all over the dough by pressing with your fingertips, being careful not to puncture all the way through.

Bake for 15 minutes, then remove from the oven and brush with the beaten egg.

Sprinkle with the remaining grape mixture and sugar.

Finish with coarse salt.

Return to the oven and continue baking until golden brown on top and crisp on the bottom, about 10 minutes.

Let the focaccia cool in the pan before cutting and serving.