Thanks for playing!
I didn’t use a secret design rule, but I think it helped that I only had time to draw a few veggie sprites so there was a limited number of pieces to work with. I made the puzzles gradually have more pieces and with a bigger board, each one after the tutorial levels has 1 more veggie than the previous one. Then I filled the board with different combinations and blocked the gaps so that it would be an exact match (I tried having a few spaces open at first but it felt too easy, I think because it left a lot of different ways to solve it). Tried several combinations of each and stuck with the one that felt the best!