Rinse the mixed berries under cold running water and drain them well or gently pat them dry with paper towels. Meanwhile, chop the walnuts with a chef's knife into coarse pieces and set them aside. Also prepare the chia seeds and pumpkin seeds in advance.
Take a large breakfast bowl and scoop the full-fat Greek yogurt into it. Use the back of a spoon to spread the yogurt evenly over the bottom, creating a smooth base for the garnish.
Sprinkle the coarse walnuts, chia seeds and pumpkin seeds evenly over the yogurt layer. Then distribute the drained mixed berries over the nuts and seeds. Finish by sprinkling a pinch of cinnamon over everything.
Serve the bowl immediately after assembling. Eat the dish right away so that the walnuts and seeds retain their crispy texture and don't become soft from the moisture of the yogurt and berries.