Line an (18 x 13-inch) half sheet pan (or other large baking pan) with parchment paper or a silicone baking mat.

In a medium bowl, whisk together the dry ingredients: flour, rye flour, and salt.

In the bowl of a standing mixer fitted with the paddle attachment, combine the yeast, ¼ cup of the water, and ½ cup of the flour mixture. Allow the mixture to rest until it is bubbling, about 10 minutes. Beat in the remaining flour mixture. Switch to the dough hook and continue beating until the mixture comes together in a rough and shaggy ball and is elastic, 5 to 6 minutes.

Transfer the dough to a large, oiled bowl; cover with plastic wrap or a damp cloth and allow the dough to rise until doubled in size, about 2 hours, at warm room temperature. The dough may be made one day, allowed to partially rise, then refrigerated overnight for use the following day or even punched down again and kept chilled for use the second day. Allow the dough to come to room temperature before proceeding.

Punch down the dough and use oiled hands to press the dough into the pan without folding it over (don’t worry if it doesn’t reach all the way into the corners). Leave the dough to rest and rise at warm room temperature for about 1 hour, or until light and puffy. Using your index finger, poke indentations over the surface of the dough about every inch or so. Brush the dough with the remaining olive oil and grind fresh black pepper over top.

Press the grapes into the dough, leaving a 1-inch border around the edges. Sprinkle the cheese and walnuts onto the dough, then mist with water from a plant mister. Allow the focaccia to rise at warm room temperature until soft and puffy, about 30 minutes.

Preheat the oven to 425°F. Bake the focaccia for 25 minutes, or until nicely browned. Remove from the oven, cool for about 10 minutes, then remove the focaccia from the pan and cut into squares to serve.

 Store loosely covered at room temperature up to 2 days or wrap tightly in aluminum foil and freeze. Reheat frozen focaccia still wrapped in foil in a preheated 375°F oven for about 20 minutes.