Thaw spinach and wring out with paper towel until very dry
Arrange bread slices in a greased 9x13 baking dish (can be smaller too)
In a medium sauce pan over medium heat add butter and shallots and cook until softened, about 3 minutes
Add garlic and tarragon and cook another 20 seconds then add spinach, cream cheese, seasonings and mix until well combined and allow creamed spinach mixture to cool almost completely
In a large bowl combine almond milk, eggs, dijon mustard
Begin by whisking creamed spinach a little at a time so the egg mixture reaches the same temperature without the eggs scrambling. Then whisk it in completely.
Pour half of the mixture over challah, then add half of the cheese
Pour the other half of mixture, then the rest of the cheese
Refrigerate for 30 minutes and preheat oven to 350F
Remove from fridge and bake for 40-50 minutes.