Slice 3 of the muffins vertically into 5 slices, and one muffin into 6 slices (vertically as in each slice has some of the crumbly topping on it.) Lay the slices in the prepared pan in a single layer, really pressing them into fit and fill any gaps.
Spread the blueberries evenly over the slices.
In a large bowl, whisk together the milk, eggs, lemon zest and a pinch of salt. Pour over the muffins evenly, cover, and refrigerate for at least 4 hours up to overnight.
Preheat your oven to 350 degrees and uncover the casserole. Make sure to really press any muffin slices that aren't covered by the egg mixture down. Bake until the eggs feel set, about 30-35 minutes.
Serve as-is or with a sprinkle of powdered sugar or a drizzle of honey and DEVOUR!