You’ll need: a day-old baguette or other crusty bread, ~6 eggs, ~1/2 cup milk (not skim for best results), a large package of frozen spinach (16oz preferable to 10oz), shredded cheddar or other cheese, bacon bits (optional), mustard (optional).
1. In a large measuring cup or mixing bowl, make the custard by whisking together the eggs and milk with a large, generous pinch (about a teaspoonful) of salt, lots of pepper, and a glob of mustard.
2. Cut the bread into cubes of ~1.5cm. Dunk into the custard and ensure that all the pieces are coated. (You can leave the custard-drenched bread in the fridge for up to several hours if you like, to make sure it soaks up all the liquid.)
3. When ready to bake, preheat oven to 350F.
4. Add about half of the bread chunks to a greased baking dish lined with parchment paper. Use a fork to press the bread down to form a denser, more even layer. Add a thin layer of cheese over the bread.
5. Next, prepare the spinach by thawing it in the microwave (a 16oz package should take about 2-3mins to warm up) and then squeezing out ALL the water. Place the dried spinach in a thick layer on top of the cheese. It may look like you have too much spinach, but use it all as it will wilt down even further as it cooks. Season the spinach with some salt. Add the bacon bits and another thin layer of cheese on top.
6. Finally, layer the remaining custardy bread on top. Compress with a fork again, and finish with a final thin layer of cheese. Bake for 40-45min or until the custard is set and the top is crispy and golden.