Combine the beer, lime juice, garlic, cumin, salt and pepper in a large zip-top plastic bag. Seal the bag and shake it to mix the marinade.
Clean the mushrooms with a damp paper towel and pull off the stems. Scrape the gills off of the underside of each mushroom and discard. Place the mushrooms in the bag with the marinade. Let them marinate at room temperature for 30 minutes, turning the bag every 5-10 minutes.
Preheat an indoor grill on high. (You can use a grill pan on medium-high or an outdoor grill too.) Brush the grates with oil and place the mushrooms on the grill. Cook for about 5 minutes on each side, until the mushrooms are tender — thicker mushrooms can take a little bit longer!
Remove the mushrooms from the grill and place them on a cutting board. Slice the mushrooms and let the liquid drain off.
Make the relish:
Toss all of the ingredients in a medium bowl. Set aside.
Make the crema:
Combine the cashews, garlic, lime juice, chipotle, and ¼ cup of water in a blender. Blend on high until the mixture is completely smooth. If needed, add additional water 1 tablespoon at a time. Once the crema is smooth, creamy, but still thick, add salt and pepper to taste.
Assemble to tacos:
Divide the sliced mushrooms onto the tortillas and top each one with relish, crema, and diced tomato, then serve.