Prepare grill for indirect heat by turning all burners to high. Mix spices and salt in small bowl.
Score cut side of eggplant with diagonal crisscross cuts about 1-inch apart, leaving skin intact. Brush tops of mushroom caps and cut-side of eggplant with oil. Sprinkle both mushrooms and eggplant evenly with 1/2 (about 4 teaspoons) of the spice mixture, making sure seasoning gets into cuts in eggplant.
Grill mushroom caps and eggplant over direct heat until charred, about 5 minutes per side. Turn off burners on one side of grill.
Place onion in center of double layer of foil. Sprinkle with about 1 1/2 teaspoons of the spice mixture. Place charred eggplant, cut side up, on top of the onions. Arrange diced tomatoes over top of the eggplant and sprinkle with about 1 1/2 teaspoons of the spice mixture. Top each eggplant half with 2 portobello mushrooms and sprinkle with remaining spice mixture. Bring up long sides of foil and fold to seal. Then fold in short sides to form a tightly sealed packet.
Place foil packet on unlit side of the grill. Cook 10 minutes. Flip packet over and cook 10 minutes longer, until eggplant is tender and can be easily separated from skin.
Carefully remove packet from grill and open foil to allow steam to escape; let cool slightly. Remove mushrooms from packet and slice into very thin strips. Peel skin from the eggplant and discard. “Shred” eggplant with two forks and mix with onions. Stir in sliced mushrooms and sprinkle mixture with lime juice.
Serve vegan barbacoa mixture in warm corn tortillas, if desired, with lime wedges and toppings such as diced tomato, pickled onions, cilantro, sliced jalapeños, vegan cheese and vegan sour cream.
Test Kitchen Tips:
• Score eggplant most of the way through, leaving the skin intact, to allow the spice mixture to get into the cuts for even flavor.
• Slicing the mushrooms into very thin strips and “shredding” the eggplant will give you a more meat-like texture.
• If you don’t have a grill, place eggplant and mushrooms on a shallow baking pan and broil on high about 5 minutes per side or until nicely charred.
• After charring the veggies and forming the foil packet as directed, you can finish cooking in the oven. Roast packets in preheated 350°F oven for about 20 minutes, flipping packets over halfway through cooking.