Heat the oil and butter in a medium skillet and cook the mushrooms, stirring, until the juices evaporate and they start to brown, about 4 minutes.
Stir in the crema, epazote, salt and pepper.
Cook, stirring, 1 minute.
Reserve off heat.
Preheat the broiler.
Lightly butter the bread, put the slices on a baking sheet and toast lightly.
Top the toasts with a portion of the mushrooms and garnish with a sprinkle of minced jalapeño.