Lightly grease a 9 x 13 inch casserole dish with butter.
Slice the loaf of brioche or challah into 1 1/2 inch thick slices. Carefully make a slit in the top of each slice (see photos). The slit should not pass all the way through the bread, but about 1-inch deep.
In a large bowl, beat the cream cheese, lemon curd, and sugar until light and fluffy, about 2-3 minutes. Spoon about 2 tablespoons of the filling into each bread slice.
In a separate bowl, vigorously whisk the milk, eggs, lemon zest, and vanilla extract until thick and creamy. Submerge each bread slice in the custard, then lay in the prepared baking dish. Repeat with remaining bread slices. Pour any leftover custard into the dish.
Cover and refrigerate the dish for 30 minutes, or up to overnight.
When ready to bake, preheat oven to 375 degrees.
Bake the French toast for 30 minutes covered, then uncover and bake 15-20 minutes longer until the tops are golden brown.
Serve the French toast with whipped cream, fresh strawberries, and maple syrup, as desired! I recommend ALL THE THINGS FUCK YEAH.