Slice 1 round of Pietree focaccia in half widthwise so you have 2 large rounds, a bottom and a top. Brush each cut side with olive or walnut oil. Spread 4 ounces cheddar cheese or 3 ounces hummus across the bottom half. Wash, peel, core, and thinly slice 2 Lodi apples. Spread those slices and the rings sliced from ¼ medium onion across the cheese or hummus. If using, sprinkle with 2 TB walnuts or pecans before putting the top half of the focaccia over the bottom.
On a hot grill, put the whole stuffed focaccia bottom side down, and grill until bread is toasted and cheese is melted. Carefully holding sides together with tongs, turn over on grill and grill 1-2 more minutes for grill marks and heating sandwich through. Serves 2-3.