Add the pecans, dates, vanilla, cinnamon and salt to the food processor, and blitz until a coarse meal forms. Process until the mixture begins to clump. It should stick together when pressed between your fingertips. Do not over-process, there should be some texture from the nuts.
Divide the mixture between four or five 4-inch (10 cm) round tart pans. Press the mixture evenly onto the base and sides, pressing with the bottom of a measuring cup to smooth things out. Place the tarts on a sheet pan and place in the freezer to firm up. If making ahead, cover the tarts first to prevent any off flavors from the freezer.
For the Berry Chia Compote:
Carefully cut any larger frozen strawberries in half so the berries are more uniform in size. Combine the berries, water, and maple syrup in a medium saucepan over medium heat. Bring to a simmer, stirring occasionally to prevent the berries from sticking. Remove from heat.
Stir in the vanilla. Sprinkle in the chia seeds and stir well. Press any clumps of chia seeds against the size of the pan and mix in. Let cool for 5 minutes. Stir one more time really well and then transfer to a sealed container. Refrigerate for up to a week, or freeze for a few months.
To Assemble the Tarts:
Just before serving time, remove the tart shells from the freezer. Fill each tart shell a third full (approximately 2 tbsp) with the berry chia compote. Spoon a few tablespoons of yogurt over the top of the berry layer, taking care not to overfill especially if adding fruit on top. Garnish with fresh berries if desired, and serve immediately.
Storage/Make-Ahead:The tart shells can be made ahead of time and kept in the freezer until serving time. The berry chia compote can also be made ahead of time and kept in the refrigerator for up to a week. Assemble the tarts just before serving time - the moisture from the berry chia compote and yogurt will soften the tart shells. *if using toasted pecans, take extra care not to over-process as the oils from the pecans will release much faster and the crust will be more delicate.**a mix of blueberries, raspberries, blackberries and strawberries