Heat olive oil in a large skillet or pan over medium heat. Add chopped onion and sauté for 3-5 minutes until softened. Add minced garlic and cook for another minute until fragrant.

Add the sliced cremini mushrooms to the skillet. Cook for 7-10 minutes, stirring occasionally, until the mushrooms release their liquid and start to brown.

Stir in the fresh spinach, ground cumin, chili powder, salt, and black pepper. Cook for 2-4 minutes, or until the spinach wilts down. Remove from heat.

While the filling is cooking, warm the corn tortillas. You can do this in a dry skillet over medium-high heat for 30 seconds per side, or wrap them in a damp paper towel and microwave for 30-60 seconds.

Assemble the tacos by spooning the spinach and mushroom filling into the warm tortillas. Top with diced avocado and fresh cilantro. Serve immediately with lime wedges.