Marinating the mushrooms: In a food processor or blender, combine 1/3 of the onion, the garlic, 3 tablespoons of the lime juice, the cumin and 1/2 teaspoon salt. Process to a smooth puree. Lay out the mushroom caps in a nonaluminum baking dish. Using a spoon, smear the marinade over both sides of each mushroom cap. Cover and let stand for 1 hour.
Preparing the salsa: Heat a gas grill to medium-high or light a charcoal fire and let it burn just until the coals are covered with gray ash and very hot. Either turn the burner(s) in the center of the grill to medium-low or bank the coals to the sides of the grill for indirect cooking. Set the cooking grate in place, cover the grill and let the grate heat up, 5 minutes or so.
Brush or spray the remaining onion slices with oil and lay in a single layer in the center (the least hot part) of the grill, along with the tomatoes. Set the chiles over the hottest part. Roast, turning everything occasionally, until the chiles' skin (but not the flesh) is blistered and uniformly blackened all over, about 5 minutes, and the onion and tomatoes are softened and browned in spots, 10 to 15 minutes, depending on their size and the heat. When the chiles are done, remove them and cover with a kitchen towel. Set the tomatoes aside on a plate. Finely chop the onion and scoop it into a bowl.
When the tomatoes are cool enough to handle, pull off their skins. Use a mortar to crush them, or place them in a food processor or blender and pulse until coarsely pureed. Add to the chopped onion.
Rub the blackened skin off the chiles, then pull out the stems and seed pods. Rinse briefly to remove any stray seeds and bits of skin. Chop into small bits and stir half into the tomato-onion mixture along with the remaining 1 tablespoon, lime juice and the cilantro. Taste, season with salt, usually about 3/4 teaspoon, and then scoop into a serving bowl.
Grilling the mushrooms: Remove the mushrooms from the marinade, spray or brush them with oil and lay gill side up over the hot part of the grill. Cook until browned in spots, about 5 minutes, then flip and move to the center of the grill - the cooler part - and continue grilling until they feel a little limp but still have some body, about 10 minutes more.
Serving the tacos: Cut the mushrooms into 1/4-inch strips. Scoop into a warm serving dish and mix with the remaining chopped poblanos. Season with salt, usually about 1/4 teaspoon. Set the mushrooms on the table along with the salsa and hot tortillas - everything you need for making wonderful soft tacos.
Working ahead: The mushroom caps can remain in their marinade for as long as 24 hours, covered in the refrigerator. The salsa will keep nicely (covered and refrigerated) for a day or so. The mushrooms are best cooked shortly before serving.