In the bowl of a stand mixer fitted with the paddle attachment, stir together water, milk, sugar, and yeast. Let sit until foamy, 5-10 minutes. Add the flour, ½ teaspoon of salt, and 2 tablespoons of olive oil and mix well on low. Attach the dough hook and raise the speed to medium-low. Allow the dough to knead for another 8 minutes.
Brush a large bowl generously with olive oil. Scrape the dough into the bowl and brush the top with more olive oil. Cover the bowl tightly with plastic wrap and let it rise in a warm place until doubled in size, about 20 minutes.
With a floured hand, punch the dough down. Turn the dough out onto a floured surface and divide it into two balls. Brush two small baking sheets with olive oil, place one ball of dough on each one, and brush the tops with more oil. Lightly cover the balls with a kitchen fowl and let rest for 20 minutes.
Dip your fingers in olive oil and press and stretch the dough balls into a large rectangle an inch or two smaller than the baking sheet. Allow your fingers to dimple the dough. Cover again with a towel and let rise in a warm place for 45 minutes to an hour.
Preheat the oven to 450ºF. Brush the tops of the dough with the remaining olive oil and sprinkle with grapes, rosemary, and remaining ½ teaspoon salt. Bake 12-15 minutes or until the crust is golden brown. Let cool slightly before cutting into large rectangles.
Preheat a panini press to medium. Place brie slices on the grape side of a slice of focaccia and top with another slice, so all the grapes are on the inside of the sandwich. If your brie is too soft to slice, simply smear it on like peanut butter.
Grill sandwiches just until cheese is melted, about 5 minutes. Allow to cool slightly and serve.