In a large bowl, combine the mushrooms. Add the vinegar, olive oil, garlic, thyme, rosemary, shallots, scallions, salt, and pepper. Marinate for 1 hour.
Prepare a hot fire in the grill.
Remove the mushrooms from the marinade and reserve the marinade for later. Place a vegetable/fish grill rack on the grate. Place the mushrooms on the rack and grill for 2 to 3 minutes per side. The portobellos may require a bit more time on the grill. When the mushrooms are cooked, transfer to a plate and let cool. Cut them into 1/2-inch pieces and transfer them with any accumulated juices to the reserved balsamic marinade. Toss to coat.
To prepare malanga, cube 2 pounds and set the remaining 1/2 pound aside. Add the 2 pounds cubed malanga to a large pot of boiling salted water and boil for 8 to 10 minutes, until the malanga is tender but still holds its shape. Drain, plunge into ice water to stop the cooking, and drain again. Transfer to the bowl with the mushrooms. Add the goat cheese and truffle oil and gently toss to mix all ingredients.
To prepare the remaining 1/2 pound malanga, slice very thin using a mandoline, any vegetable slicer, or box grater. Cut the slices crosswise into fine strips with a sharp knife.
In a medium-size saute pan, heat the oil to 350 degrees and fry the malanga until golden brown and crisp. Transfer to paper towels to drain.
To serve, spoon the mushroom salad over the tomato slices and sprinkle with the fried malanga.
Recipe courtesy Douglas Rodriguez, author, Latin Flavors on the Grill