These squares offer a lovely colourful “pop” to an hors d’oeuvre plate, and the combination of spinach, mushrooms and cheese works wonderfully well.
Make ahead: These squares are a real timesaver when you make them ahead and, before cutting them, chill for at least two hours or freeze for up to three months. Thaw overnight in the fridge, then slice while still cold. Before serving, place on a baking tray and warm in the oven at 325°F (160°C) for about 12 minutes.
Note: These squares also make a great base for fancier hors d’oeuvres. Bake them in a 22.5 × 32.5 centimeter pan (bake time stays the same) so that they are a little flatter. Once baked, top each square with a cooked shrimp and a little mayo, or some smoked salmon with dill cream cheese, or roasted peppers and a bocconcini, secured with a frilly toothpick.