Tuna and Spinach Bake

adapted from The Low-Carb Gourmet by Karen Barnaby

2 T butter
1/2 cup finely chopped onion (I used a whole onion)
the last few stalks from the core of a celery, chopped, with the leaves (my addition)
2 10-oz bags ready-to-use spinach (I had most of a 16 oz box left, so I just used that much)
2 6-oz cans chunk light (or solid albicore) tuna, drained and flaked
3/4 cup whipping cream
3 eggs
1/4-1/2 t salt
4 oz cheese, shredded (originally aged cheddar.  I used about 6 oz--part anejo and part cheddar, because they needed to get used up.)

Preheat oven to 350.
Melt the butter in a large frying pan over medium heat.  (I don't have a frying pan that big, so I used a wok.)  Add the onion and celery and saute until soft, about 2 minutes.  Add the spinach and cook until wilted.  (This is where you want the biggest frying pan/wok available.  I had to add the spinach in stages as it started to wilt.)
Spread the spinach mixture in an 8x8" baking dish.  Top with the tuna.
Beat the cream, eggs, and salt together with a fork.  Pour over the tuna.
Sprinkle the cheese evenly over the top.

Bake for 40 minutes, or until set and the top is lightly browned.