Place the chopped fruit, sugar (or honey/maple syrup) and lemon juice into a deep skillet or pot and allow to rest for 30-45 minutes for some of the juices to release.
Turn on the heat, bring to a boil and then simmer for 15-20 minutes. Remove from heat and stir in the vanilla, and the chia seeds or cornstarch (if using.) Allow to cool in fridge or freezer while you make the crumble. You need about 1 full cup of jam for the recipe, so save any extra for ice cream topping. (Don't have time to make jam from scratch? Just use 1 cup of your favorite store-bought jam.)
Turn your oven to 350 and line the bottom of a 9x9 (for thick bars) or 9x13 (for thinner bars) baking dish with parchment paper.
Mix together the oats, the spelt flour, the melted and cooled butter or oil, the baking soda and the salt in a bowl. Stir till it's crumbly, then pour in 1/2 cup of maple syrup and stir till sticky and clumping together.
Press 2/3 - 3/4 of the oat crumble mixture onto the bottom of your baking dish. Place a layer of parchment over the crumble and press down firmly to make smooth.
Spread about a cup or a little more of the jam in a thin layer over top the oat base, and then crumble the remaining oat mixture over top.
Bake for 25-30 minutes until done. Check after 20 and cover with tin foil if the top browns too fast. Allow to cool completely before cutting (20-30 minutes, maybe even with a few minutes in the fridge.) Extras keep well in the fridge or freezer (wrapped individually.)
