Make the jam by combining the raspberries, fruit syrup and lemon juice in a small saucepan. Bring the mixture to a medium-high heat and cook, stirring frequently, until the raspberries have broken down. Remove from heat and stir in the chia seeds. Place in the fridge for 1 hour to thicken.
Make the cookie dough by adding the almond flour, gluten-free oat flour, fruit syrup and coconut oil to a bowl. Mix in the bowl with a wooden spoon, then transfer the dough to a lightly floured surface and knead until uniform. Divide the dough into two, cover each piece with plastic wrap and place in the fridge for at least 20 minutes.
Preheat oven to 170°C/325°F/gas mark 3. Line two baking sheets with parchment paper. Working one piece of dough at a time, roll the dough to ¼ inch thick between two sheets of parchment paper. Using a cookie cutter, cut an equal amount of cookies with a shape you desire cut out of the middle, such as a heart, star or triangle (this will be the top cookie), and with no shape cut out (this will be the bottom cookie). Transfer to the lined baking sheet and refrigerate for at least 15 minutes.
Bake the cookies in the oven for 8-12 minutes, until very slightly browned. Transfer to a cookie sheet and cool completely. Add about 1 teaspoon of raspberry jam to the bottom cookie. Gently sandwich the tops and bottoms together.