This is a staple in our house. I hope you love it as much as we do!

Let's start with the fish. I usually use tilapia or flounder, but any white fish will do:

I cut these fillets in half lengthwise and horizontally, so I end up with 8 pieces of fish per fillet. I find this is easier to fit into the tortilla later.
For the batter you will need the following ingredients:

1/2 cup flour
1/2 cup seltzer water (or club soda would work, too)
1/2 teaspoon baking powder
a dash (or two) of hot sauce
You may need to add more seltzer if your batter is too thick. Dip each piece of fish into the batter and then into panko breadcrumbs. You want a nice, tick coating.
Heat about 1-1 1/2 inches of oil in a pan. I re-use my oil, so that's why it looks so dark.

I don't really test the temperature of my oil, but I do test it by dropping pieces of batter and breadcrumbs into the oil. When they sizzle on the surface, it's ready!
Fry your fish in batches so you don't overcrowd the pan.

Now for the sauce. This is my favorite part!

You will need:
4 tomatillos
2 jalapenos (less or more to taste)
half an onion
1 ripe avocado
a bunch of cilantro
2 garlic cloves
about 1 tablespoon olive oil
Optional (not pictured):
Juice of 1 lime
Pinch of salt

Puree all ingredients together in a blender. Things will go a little easier if you start with the tomatillos and onion and end with the cilantro (and your blender will probably thank you - we learned this the hard way).
It should look something like this:

Keeping the avocado pit in the sauce helps keep it nice and green (a little bit of lime juice helps, too). This is also great on eggs!
Lastly, let's fry up some corn tortillas.

I dip them in the leftover fry oil (healthy!) and then fry them in a shallow pan.

Lay the fried tortillas over an old paper towel tube covered in foil to help get the folded shape.
And that's it! Garnish with chopped cilantro, lettuce, diced onions, and plenty of green sauce. Enjoy!