Preheat the oven to 400 degrees.

Rub a little olive oil on the top side of the portobellas.  Sprinkle with salt and freshly ground black pepper and place them, top side down, on a rimmed baking sheet. The stem side should be up on the baking sheet.  (But the stem should be removed, of course!)  Next, drizzle each mushroom with ~1/4 teaspoon minced garlic and 1 teaspoon  balsamic vinegar.  Use your fingers or the back side of  a spoon to evenly spread the garlic and vinegar on the surface of the ‘shroom.  Cover with foil and bake in the preheated oven for 20 minutes.  Remove the foil and allow to roast an additional 10 minutes.

Meanwhile, prepare the chutney:

Heat the oil in a medium saucepan.  Add the apple, agave nectar, raisins, cranberries, cinnamon, nutmeg, lemon juice, and apple cider.  Cook over medium heat until the apple is tender, the dried fruit softens, and the cider boils away, about 15 minutes.

In a small skillet, heat the chopped walnuts over medium-low heat.  Shake the pan occasionally until the walnuts are lightly toasted, about 5 minutes.

When the portobella caps are ready, remove them from the oven and transfer to their plates.  Fill each cap with 1/4 of the chutney and then sprinkle with the toasted walnuts.