Combine berries, lemon juice, and maple syrup and stir gently. Let set for 2 hours (or overnight) – I never think to do this part ahead like I'm supposed to, though, and just skip that step and it's still good. Strain the berries, reserving juices. Mash or puree half the berries and set aside. (If you have frozen berries, cook until soft, cut in half, then strain, etc.)