Warm oil in a skillet. Add mushrooms and cook briefly, then add garlic and cook until soft. Add dried epazote if using; stir. Remove mixture from the skillet.

Using the same skillet, melt butter. Add a tortilla, then add queso Oaxaca to one half. Top with half the mushroom mixture, half the squash blossoms, fresh epazote leaves if using, and half the cilantro. Fold tortilla over. Cook until melty and toasted. Repeat with second tortilla.

Slice into wedges and serve with salsa verde and sour cream.