Preheat the oven to 350°F (175°C).

Line the bottom and two sides of an 8x8-inch square pan with parchment paper. Lightly grease the pan and parchment paper.

In a large mixing bowl, stir together the oats, mixed seeds, dried fruit, salt, and cinnamon.

In a medium saucepan, heat the butter, brown sugar, and golden syrup over low heat until the butter melts.

Stir the mixture with a whisk or silicone spatula until the brown sugar dissolves and the mixture comes together, about 30 seconds.

Pour the butter mixture over the oats. Use a silicone spatula to stir everything together, ensuring all the dry ingredients are well coated with the syrup mixture.

Spread the oat mixture in an even layer in the prepared pan. Compact the oats by pressing down the top with a spatula or the bottom of a glass.

Bake the flapjacks until they turn golden brown around the edges and lightly golden in the middle, about 25 to 30 minutes.

For the cleanest slices, let the flapjacks cool completely, at least 1 to 2 hours, before using the parchment paper to lift them out of the pan.

Use a serrated bread knife to slice into 9 large or 16 small flapjacks.