Preheat a lightly oiled grill on medium-high heat.
While the grill is heating up, prepare the pico de gallo. In a small mixing bowl, combine tomatoes, onion, cilantro, garlic, salt and lime juice. Set aside.
Next, prepare the grilled Portobello filling. Remove the stems from the Portobello, and discard. Using a spoon, scrape out the dark gills from the underside of the mushroom caps.
Lightly brush the Portobello and the onions on both sides with canola oil, and sprinkle generously with salt and pepper.
Arrange the mushrooms, onion, and poblano on the hot grill, and cook for 5-6 minutes per side or until the mushrooms are softened and the onion is lightly charred. Remove from heat and transfer to a cutting board.
Remove the stem and seeds from the poblano, and discard. Cut the poblano and the Portobello into thin strips, and roughly chop the onion. Transfer to a mixing bowl and toss with lime juice and chipotle powder until well coated.
To assemble, arrange 2 tortillas per plate, and divide the mushroom filling evenly between them. Serve with pico de gallo and bowls of the different toppings so that everyone can garnish their tacos as they please.