Tidbit: Panzanella, stale bread salad, requires that you soak and wring out the bread before assembling the salad. If you make this with fresh crusty bread or the bread has staled a bit but not hardened, the juice from the tomatoes should be enough to tenderize the bread's crusty edges.
Preheat grill to medium-high and oven to 425 degrees F.
Using a damp towel, brush mushroom caps to clean. Using a pastry brush, coat the caps with extra-virgin olive oil and grill with lid down or roast about 5 to 6 minutes, turning once, until tender but still firm at center.
Wring the spinach out in a clean kitchen towel. Separate the spinach with your fingers and combine with artichokes, ricotta cheese, egg yolk, grated garlic, salt and pepper, to taste, and nutmeg.
Remove mushroom caps from grill.
To portion-control the filling, divided the stuffing into 4 parts then just split each of those evenly between 2 caps. Sprinkle the caps with Parmigiano and cook 5 to 6 minutes more in the preheated oven to set filling and lightly brown tops.
Do not seed tomatoes. Chop and place in a salad bowl. Add basil, bread, onions, 1/4 cup EVOO, lots of salt and pepper and toss with your hands to combine.
Serve 2 caps with a pile of salad alongside, if desired, per person.