Preheat the oven to 425°F. In a glass liquid measure or glass jar, combine 6 Tbs. of the olive oil, the vinegar, the honey, 2 tsp. of the rosemary, the garlic, and 1/2 tsp salt. Whisk or shake well to combine.
Rub a shallow 11-by-7-inch baking dish with olive oil and combine the grapes and the shallots and season with a big pinch of salt. Rub a 13-by-9-inch baking dish with olive oil and arrange the portobello caps, stem-side up, in a single layer and season each cap with a pinch of salt. Pour 1 Tbs. of the dressing inside and around the edges of each of the mushroom caps (for a total of 4 Tbs.). Let the mushrooms marinate for 20 minutes.
Drizzle 2 Tbs. of the dressing over the grapes and shallots and toss to coat. (Reserve the remaining dressing.) Roast the grapes and shallots, stirring once or twice, until soft and browned, about 40 minutes. (The liquid on the bottom of the pan will be somewhat syrupy.)
Meanwhile, fill a large pasta pot three-quarters full of water and bring to a boil. Rinse the farro in a colander. Add 1 tsp. salt and the farro to the boiling water and cook at a rapid simmer or low boil, partially covered, until the grains are tender (they will look “popped”), about 30 minutes. Drain in a colander, return to the pot (off the heat), and cover to keep warm.
When the grapes and shallots have been roasting for 20 minutes, put the portobello caps in the oven and roast for 20 minutes. (The edges of the mushrooms may have flattened out; they will have lost some volume; and liquid may be bubbling on top of them. The bottoms will be brown, too, and they will be cooked through.)
Transfer each of the mushroom caps, keeping them stem-side up, to an individual warm plate or a warm platter. (If there are any juices remaining in the pan, drizzle them over the mushrooms.) Transfer the farro to a medium mixing bowl and season it with 1/4 tsp. salt. Stir in the roasted grapes and shallots (be sure to scrape out all of the pan juices), the remaining 1 tsp. rosemary, and 1 to 2 Tbs. of the remaining dressing. Taste and season with more salt, if necessary. Spoon the mixture evenly over each of the mushrooms. (For a nicer presentation, use a smaller amount of the farro mixture and
mound it completely in the center of the mushroom with just the edges of
the mushroom peeking out and the rosemary sprig leaning against the
mound. You’ll have extra farro, but it reheats well.) Garnish each serving with a rosemary sprig and serve right away.