Preheat oven to 350F. Grease a shallow baking dish about 9x13-ish-oval (or close, such as 8x11 rectangle etc.)
Cut croissants in half. Place the bottoms of the croissants over the bottom of your pan, trying to fill as much of the bottom evenly as possible, overlapping just slightly, if necessary. Place the top halves of the croissants randomly overtop.
Prepare the custard by combining all the custard ingredients in a medium bowl and whisking well to combine. Pour over croissants in pan. (Liquid should come up a little more than halfway, leaving most of the croissant tops exposed). Let stand 5-10 minutes, while you prepare the topping.
Prepare the topping by combining the butter and brown sugar. Cut the butter into the brown sugar with a pastry cutter or just use your fingers to rub the butter into the brown sugar until you have a crumb mixture. Scatter the crumb mixture overtop of the croissants, focusing on the areas between the croissant tops. Scatter the pecan pieces overtop as well.
Bake in preheated oven for about 45-50 minutes, checking at the 35-40 minute mark and loosely covering the top with a sheet of aluminum foil, if the croissant tops are getting too brown. Be sure to test for doneness by inserting a spoon and lifting to make sure the bottom is moderately set, as it make look done due to the brown top, but the underneath may still be runny. When filling is set, remove from oven and let stand 5-10 minutes before serving warm. Nice with a scoop of vanilla ice cream!