Preheat the oven to 200C /400F.
In a large, deep saucepan, heat a couple tablespoons of olive oil and saute your onion slices on a medium heat until soft and starting to brown.
1 small red onion, 2 Tbsp olive oil
Now cook the mushrooms. If you're using a normal sized large frying pan, you will want to do this in two batches to avoid overcrowding the pan. So cook half the mushrooms with the onions, then pour them into a bowl, quickly cook the rest of the mushrooms and then add the first batch back into the pan. (If you're using an extra large pan and feel you can fit all your mushrooms in at once, go for it.) Deglaze the pan with water or white wine if things are sticking.
500 g mushrooms
Add the garlic and tarragon. Cook until fragrant, just a minute or so, and then remove from the heat.
3 cloves garlic, 1 Tbsp dried tarragon
Add the mascarpone to the pan, stir through to melt.
3 Tbsp mascarpone cheese
Finally, add the crumbled feta and mix until just combined (we're not trying to get the feta to melt, we want it to stay a little chunky).
35 g feta, cotija or queso fresco cheese
Now spoon the filling into the corn tortillas, and roll them up.
6-8 corn tortillas
Pour a little bit of the enchilada sauce on the bottom of a casserole pan (only enough to cover the bottom - no more) and place your rolled enchiladas on top, folded side down.
1 portion enchilada sauce
Cover with the remaining sauce.
Sprinkle the grated mozzarella on top.
100 g grated mozzarella
Bake for 20-25 minutes, uncovered, until the cheese is browned to your liking.
Add a little more crumbled feta, the chopped coriander (cilantro) and any other toppings you're using.
Optional toppings: avocado or guacamole, sour cream, chili slices, lime wedges, 1 handful chopped fresh coriander (cilantro)