I used an 8" square Pyrex dish for my buckle, with 2 small ramekins on the side for tasters as the large dish was headed off to sister-in-law's Friday morning Tickling Tech session for teachers. A mixture of sugar, cornstarch, lemon zest, a bit of salt, and lemon juice goes in the pan and mixed, then in go the blueberries to get coated with it. I spooned out a bit for each ramekin at this point. Then came a quick cake batter, mixed in the fats-into-flour method, then beating in an egg mixture. The dough got dolloped onto the ramekins and the square pan, leaving a border around the edges and a center hole in the big pan. I didn't make my center hole the suggested 2 inches (with some vague thought that my square pan might not need that large a hole, or maybe it was thinking that batter was piled up pretty good around the sides, and it closed up in the baking. I suspect that just slowed the baking down a bit, as that center part was the last to get done.
The little ramekins were done in 20 minutes. The big dish took the full 40 minutes, covered with foil to slow down the browning for the last 10. I ate my little ramekin of buckle while waiting for the larger one to finish, warm, and it was lovely with the cake topping on sweet fruit. I generally will make a crumble when I want a baked fruit dessert because I like the crunchy character of that topping, but this was a nice change.