We got this recipe from Smoke and Spice: The Real Way to Barbecue by Cheryl and Bill Jamison.
12 ounces portobello mushrooms
Coarse Kosher or sea salt
2 Roma or Italian plum tomatoes, seeded and chopped
1/3 cup extra-virgin olive oil
1 tablespoon balsamic vinegar
1 tablespoon chopped fresh basil
1 tablespoon minced fresh parsley
3 garlic cloves, minced
2 scallions, sliced
Salt and coarsely ground black pepper to taste
1 large bag of spinach, or equivalent
Crumbled feta cheese, optional
Sunflower seeds, optional
I've done this in my barbecue/smoker, but I guess you could use a covered grill using low, indirect heat.
Slice the mushrooms into large, bite-size pieces and salt them lightly. Arrange mushroom pieces on a small grill pan, rack, or a piece of heavy-duty aluminium foil.
Place in the barbecue and cook for 15 to 20 minutes, until they ooze liquid and are cooked through.
While the mushrooms are cooking, mix the dressing ingredients.
Add the cooked mushrooms and mix lightly. Can be served warm or chilled.
Place a layer of spinach on a serving platter. Spoon the dressing over the spinach. Garnish with feta cheese and sunflower seeds, if desired. Alternatively, serve the cheese and seeds separately and let folks add them if they'd like.